[p5sagit/p5-mst-13.2.git] / pod / perltodo.pod

=head1 NAME

perltodo - Perl TO-DO List

=head1 DESCRIPTION

This is a list of wishes for Perl. The tasks we think are smaller or easier
are listed first. Anyone is welcome to work on any of these, but it's a good
idea to first contact I<perl5-porters@perl.org> to avoid duplication of
effort. By all means contact a pumpking privately first if you prefer.

Whilst patches to make the list shorter are most welcome, ideas to add to
the list are also encouraged. Check the perl5-porters archives for past
ideas, and any discussion about them. One set of archives may be found at:

    http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/

What can we offer you in return? Fame, fortune, and everlasting glory? Maybe
not, but if your patch is incorporated, then we'll add your name to the
F<AUTHORS> file, which ships in the official distribution. How many other
programming languages offer you 1 line of immortality?

=head1 Tasks that only need Perl knowledge

=head2 merge common code in installperl and installman

There are some common subroutines and a common C<BEGIN> block in F<installperl>
and F<installman>. These should probably be merged. It would also be good to
check for duplication in all the utility scripts supplied in the source
tarball. It might be good to move them all to a subdirectory, but this would
require careful checking to find all places that call them, and change those
correctly.

=head2 common test code for timed bail out

Write portable self destruct code for tests to stop them burning CPU in
infinite loops. This needs to avoid using alarm, as some of the tests are
testing alarm/sleep or timers.

=head2 POD -E<gt> HTML conversion in the core still sucks

Which is crazy given just how simple POD purports to be, and how simple HTML
can be. It's not actually I<as> simple as it sounds, particularly with the
flexibility POD allows for C<=item>, but it would be good to improve the
visual appeal of the HTML generated, and to avoid it having any validation
errors. See also L</make HTML install work>, as the layout of installation tree
is needed to improve the cross-linking.

The addition of C<Pod::Simple> and its related modules may make this task
easier to complete.

=head2 merge checkpods and podchecker

F<pod/checkpods.PL> (and C<make check> in the F<pod/> subdirectory)
implements a very basic check for pod files, but the errors it discovers
aren't found by podchecker. Add this check to podchecker, get rid of
checkpods and have C<make check> use podchecker.

=head2 Parallel testing

(This probably impacts much more than the core: also the Test::Harness
and TAP::* modules on CPAN.)

The core regression test suite is getting ever more comprehensive, which has
the side effect that it takes longer to run. This isn't so good. Investigate
whether it would be feasible to give the harness script the B<option> of
running sets of tests in parallel. This would be useful for tests in
F<t/op/*.t> and F<t/uni/*.t> and maybe some sets of tests in F<lib/>.

Questions to answer

=over 4

=item 1

How does screen layout work when you're running more than one test?

=item 2

How does the caller of test specify how many tests to run in parallel?

=item 3

How do setup/teardown tests identify themselves?

=back

Pugs already does parallel testing - can their approach be re-used?

=head2 Make Schwern poorer

We should have tests for everything. When all the core's modules are tested,
Schwern has promised to donate to $500 to TPF. We may need volunteers to
hold him upside down and shake vigorously in order to actually extract the
cash.

=head2 Improve the coverage of the core tests

Use Devel::Cover to ascertain the core modules's test coverage, then add
tests that are currently missing.

=head2 test B

A full test suite for the B module would be nice.

=head2 Deparse inlined constants

Code such as this

    use constant PI => 4;
    warn PI

will currently deparse as

    use constant ('PI', 4);
    warn 4;

because the tokenizer inlines the value of the constant subroutine C<PI>.
This allows various compile time optimisations, such as constant folding
and dead code elimination. Where these haven't happened (such as the example
above) it ought be possible to make B::Deparse work out the name of the
original constant, because just enough information survives in the symbol
table to do this. Specifically, the same scalar is used for the constant in
the optree as is used for the constant subroutine, so by iterating over all
symbol tables and generating a mapping of SV address to constant name, it
would be possible to provide B::Deparse with this functionality.

=head2 A decent benchmark

C<perlbench> seems impervious to any recent changes made to the perl core. It
would be useful to have a reasonable general benchmarking suite that roughly
represented what current perl programs do, and measurably reported whether
tweaks to the core improve, degrade or don't really affect performance, to
guide people attempting to optimise the guts of perl. Gisle would welcome
new tests for perlbench.

=head2 fix tainting bugs

Fix the bugs revealed by running the test suite with the C<-t> switch (via
C<make test.taintwarn>).

=head2 Dual life everything

As part of the "dists" plan, anything that doesn't belong in the smallest perl
distribution needs to be dual lifed. Anything else can be too. Figure out what
changes would be needed to package that module and its tests up for CPAN, and
do so. Test it with older perl releases, and fix the problems you find.

To make a minimal perl distribution, it's useful to look at
F<t/lib/commonsense.t>.

=head2 Improving C<threads::shared>

Investigate whether C<threads::shared> could share aggregates properly with
only Perl level changes to shared.pm

=head2 POSIX memory footprint

Ilya observed that use POSIX; eats memory like there's no tomorrow, and at
various times worked to cut it down. There is probably still fat to cut out -
for example POSIX passes Exporter some very memory hungry data structures.

=head2 embed.pl/makedef.pl

There is a script F<embed.pl> that generates several header files to prefix
all of Perl's symbols in a consistent way, to provide some semblance of
namespace support in C<C>. Functions are declared in F<embed.fnc>, variables
in F<interpvar.h>. Quite a few of the functions and variables
are conditionally declared there, using C<#ifdef>. However, F<embed.pl>
doesn't understand the C macros, so the rules about which symbols are present
when is duplicated in F<makedef.pl>. Writing things twice is bad, m'kay.
It would be good to teach C<embed.pl> to understand the conditional
compilation, and hence remove the duplication, and the mistakes it has caused.

=head2 use strict; and AutoLoad

Currently if you write

    package Whack;
    use AutoLoader 'AUTOLOAD';
    use strict;
    1;
    __END__
    sub bloop {
        print join (' ', No, strict, here), "!\n";
    }

then C<use strict;> isn't in force within the autoloaded subroutines. It would
be more consistent (and less surprising) to arrange for all lexical pragmas
in force at the __END__ block to be in force within each autoloaded subroutine.

There's a similar problem with SelfLoader.

=head1 Tasks that need a little sysadmin-type knowledge

Or if you prefer, tasks that you would learn from, and broaden your skills
base...

=head2 make HTML install work

There is an C<installhtml> target in the Makefile. It's marked as
"experimental". It would be good to get this tested, make it work reliably, and
remove the "experimental" tag. This would include

=over 4

=item 1

Checking that cross linking between various parts of the documentation works.
In particular that links work between the modules (files with POD in F<lib/>)
and the core documentation (files in F<pod/>)

=item 2

Work out how to split C<perlfunc> into chunks, preferably one per function
group, preferably with general case code that could be used elsewhere.
Challenges here are correctly identifying the groups of functions that go
together, and making the right named external cross-links point to the right
page. Things to be aware of are C<-X>, groups such as C<getpwnam> to
C<endservent>, two or more C<=items> giving the different parameter lists, such
as

    =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
    =item substr EXPR,OFFSET,LENGTH
    =item substr EXPR,OFFSET

and different parameter lists having different meanings. (eg C<select>)

=back

=head2 compressed man pages

Be able to install them. This would probably need a configure test to see how
the system does compressed man pages (same directory/different directory?
same filename/different filename), as well as tweaking the F<installman> script
to compress as necessary.

=head2 Add a code coverage target to the Makefile

Make it easy for anyone to run Devel::Cover on the core's tests. The steps
to do this manually are roughly

=over 4

=item *

do a normal C<Configure>, but include Devel::Cover as a module to install
(see F<INSTALL> for how to do this)

=item *

    make perl

=item *

    cd t; HARNESS_PERL_SWITCHES=-MDevel::Cover ./perl -I../lib harness

=item *

Process the resulting Devel::Cover database

=back

This just give you the coverage of the F<.pm>s. To also get the C level
coverage you need to

=over 4

=item *

Additionally tell C<Configure> to use the appropriate C compiler flags for
C<gcov>

=item *

    make perl.gcov

(instead of C<make perl>)

=item *

After running the tests run C<gcov> to generate all the F<.gcov> files.
(Including down in the subdirectories of F<ext/>

=item *

(From the top level perl directory) run C<gcov2perl> on all the C<.gcov> files
to get their stats into the cover_db directory.

=item *

Then process the Devel::Cover database

=back

It would be good to add a single switch to C<Configure> to specify that you
wanted to perform perl level coverage, and another to specify C level
coverage, and have C<Configure> and the F<Makefile> do all the right things
automatically.

=head2 Make Config.pm cope with differences between built and installed perl

Quite often vendors ship a perl binary compiled with their (pay-for)
compilers.  People install a free compiler, such as gcc. To work out how to
build extensions, Perl interrogates C<%Config>, so in this situation
C<%Config> describes compilers that aren't there, and extension building
fails. This forces people into choosing between re-compiling perl themselves
using the compiler they have, or only using modules that the vendor ships.

It would be good to find a way teach C<Config.pm> about the installation setup,
possibly involving probing at install time or later, so that the C<%Config> in
a binary distribution better describes the installed machine, when the
installed machine differs from the build machine in some significant way.

=head2 linker specification files

Some platforms mandate that you provide a list of a shared library's external
symbols to the linker, so the core already has the infrastructure in place to
do this for generating shared perl libraries. My understanding is that the
GNU toolchain can accept an optional linker specification file, and restrict
visibility just to symbols declared in that file. It would be good to extend
F<makedef.pl> to support this format, and to provide a means within
C<Configure> to enable it. This would allow Unix users to test that the
export list is correct, and to build a perl that does not pollute the global
namespace with private symbols.

=head2 Cross-compile support

Currently C<Configure> understands C<-Dusecrosscompile> option. This option
arranges for building C<miniperl> for TARGET machine, so this C<miniperl> is
assumed then to be copied to TARGET machine and used as a replacement of full
C<perl> executable.

This could be done little differently. Namely C<miniperl> should be built for
HOST and then full C<perl> with extensions should be compiled for TARGET.
This, however, might require extra trickery for %Config: we have one config
first for HOST and then another for TARGET.  Tools like MakeMaker will be
mightily confused.  Having around two different types of executables and
libraries (HOST and TARGET) makes life interesting for Makefiles and
shell (and Perl) scripts.  There is $Config{run}, normally empty, which
can be used as an execution wrapper.  Also note that in some
cross-compilation/execution environments the HOST and the TARGET do
not see the same filesystem(s), the $Config{run} may need to do some
file/directory copying back and forth.

=head2 roffitall

Make F<pod/roffitall> be updated by F<pod/buildtoc>.

=head1 Tasks that need a little C knowledge

These tasks would need a little C knowledge, but don't need any specific
background or experience with XS, or how the Perl interpreter works

=head2 Modernize the order of directories in @INC

The way @INC is laid out by default, one cannot upgrade core (dual-life)
modules without overwriting files. This causes problems for binary
package builders.  One possible proposal is laid out in this
message:
L<http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2002-04/msg02380.html>.

=head2 -Duse32bit*

Natively 64-bit systems need neither -Duse64bitint nor -Duse64bitall.
On these systems, it might be the default compilation mode, and there
is currently no guarantee that passing no use64bitall option to the
Configure process will build a 32bit perl. Implementing -Duse32bit*
options would be nice for perl 5.12.

=head2 Make it clear from -v if this is the exact official release

Currently perl from C<p4>/C<rsync> ships with a F<patchlevel.h> file that
usually defines one local patch, of the form "MAINT12345" or "RC1". The output
of perl -v doesn't report that a perl isn't an official release, and this
information can get lost in bugs reports. Because of this, the minor version
isn't bumped up until RC time, to minimise the possibility of versions of perl
escaping that believe themselves to be newer than they actually are.

It would be useful to find an elegant way to have the "this is an interim
maintenance release" or "this is a release candidate" in the terse -v output,
and have it so that it's easy for the pumpking to remove this just as the
release tarball is rolled up. This way the version pulled out of rsync would
always say "I'm a development release" and it would be safe to bump the
reported minor version as soon as a release ships, which would aid perl
developers.

This task is really about thinking of an elegant way to arrange the C source
such that it's trivial for the Pumpking to flag "this is an official release"
when making a tarball, yet leave the default source saying "I'm not the
official release".

=head2 Profile Perl - am I hot or not?

The Perl source code is stable enough that it makes sense to profile it,
identify and optimise the hotspots. It would be good to measure the
performance of the Perl interpreter using free tools such as cachegrind,
gprof, and dtrace, and work to reduce the bottlenecks they reveal.

As part of this, the idea of F<pp_hot.c> is that it contains the I<hot> ops,
the ops that are most commonly used. The idea is that by grouping them, their
object code will be adjacent in the executable, so they have a greater chance
of already being in the CPU cache (or swapped in) due to being near another op
already in use.

Except that it's not clear if these really are the most commonly used ops. So
as part of exercising your skills with coverage and profiling tools you might
want to determine what ops I<really> are the most commonly used. And in turn
suggest evictions and promotions to achieve a better F<pp_hot.c>.

=head2 Allocate OPs from arenas

Currently all new OP structures are individually malloc()ed and free()d.
All C<malloc> implementations have space overheads, and are now as fast as
custom allocates so it would both use less memory and less CPU to allocate
the various OP structures from arenas. The SV arena code can probably be
re-used for this.

Note that Configuring perl with C<-Accflags=-DPL_OP_SLAB_ALLOC> will use
Perl_Slab_alloc() to pack optrees into a contiguous block, which is
probably superior to the use of OP arenas, esp. from a cache locality
standpoint.  See L<Profile Perl - am I hot or not?>.

=head2 Improve win32/wince.c

Currently, numerous functions look virtually, if not completely,
identical in both C<win32/wince.c> and C<win32/win32.c> files, which can't
be good.

=head2 Use secure CRT functions when building with VC8 on Win32

Visual C++ 2005 (VC++ 8.x) deprecated a number of CRT functions on the basis
that they were "unsafe" and introduced differently named secure versions of
them as replacements, e.g. instead of writing

    FILE* f = fopen(__FILE__, "r");

one should now write

    FILE* f;
    errno_t err = fopen_s(&f, __FILE__, "r"); 

Currently, the warnings about these deprecations have been disabled by adding
-D_CRT_SECURE_NO_DEPRECATE to the CFLAGS. It would be nice to remove that
warning suppressant and actually make use of the new secure CRT functions.

There is also a similar issue with POSIX CRT function names like fileno having
been deprecated in favour of ISO C++ conformant names like _fileno. These
warnings are also currently suppressed by adding -D_CRT_NONSTDC_NO_DEPRECATE. It
might be nice to do as Microsoft suggest here too, although, unlike the secure
functions issue, there is presumably little or no benefit in this case.

=head2 __FUNCTION__ for MSVC-pre-7.0

Jarkko notes that one can things morally equivalent to C<__FUNCTION__>
(or C<__func__>) even in MSVC-pre-7.0, contrary to popular belief.
See L<http://www.codeproject.com/debug/extendedtrace.asp> if you feel like
making C<PERL_MEM_LOG> more useful on Win32.

=head1 Tasks that need a knowledge of XS

These tasks would need C knowledge, and roughly the level of knowledge of
the perl API that comes from writing modules that use XS to interface to
C.

=head2 autovivification

Make all autovivification consistent w.r.t LVALUE/RVALUE and strict/no strict;

This task is incremental - even a little bit of work on it will help.

=head2 Unicode in Filenames

chdir, chmod, chown, chroot, exec, glob, link, lstat, mkdir, open,
opendir, qx, readdir, readlink, rename, rmdir, stat, symlink, sysopen,
system, truncate, unlink, utime, -X.  All these could potentially accept
Unicode filenames either as input or output (and in the case of system
and qx Unicode in general, as input or output to/from the shell).
Whether a filesystem - an operating system pair understands Unicode in
filenames varies.

Known combinations that have some level of understanding include
Microsoft NTFS, Apple HFS+ (In Mac OS 9 and X) and Apple UFS (in Mac
OS X), NFS v4 is rumored to be Unicode, and of course Plan 9.  How to
create Unicode filenames, what forms of Unicode are accepted and used
(UCS-2, UTF-16, UTF-8), what (if any) is the normalization form used,
and so on, varies.  Finding the right level of interfacing to Perl
requires some thought.  Remember that an OS does not implicate a
filesystem.

(The Windows -C command flag "wide API support" has been at least
temporarily retired in 5.8.1, and the -C has been repurposed, see
L<perlrun>.)

Most probably the right way to do this would be this:
L</"Virtualize operating system access">.

=head2 Unicode in %ENV

Currently the %ENV entries are always byte strings.
See L</"Virtualize operating system access">.

=head2 Unicode and glob()

Currently glob patterns and filenames returned from File::Glob::glob()
are always byte strings.  See L</"Virtualize operating system access">.

=head2 Unicode and lc/uc operators

Some built-in operators (C<lc>, C<uc>, etc.) behave differently, based on
what the internal encoding of their argument is. That should not be the
case. Maybe add a pragma to switch behaviour.

=head2 use less 'memory'

Investigate trade offs to switch out perl's choices on memory usage.
Particularly perl should be able to give memory back.

This task is incremental - even a little bit of work on it will help.

=head2 Re-implement C<:unique> in a way that is actually thread-safe

The old implementation made bad assumptions on several levels. A good 90%
solution might be just to make C<:unique> work to share the string buffer
of SvPVs. That way large constant strings can be shared between ithreads,
such as the configuration information in F<Config>.

=head2 Make tainting consistent

Tainting would be easier to use if it didn't take documented shortcuts and
allow taint to "leak" everywhere within an expression.

=head2 readpipe(LIST)

system() accepts a LIST syntax (and a PROGRAM LIST syntax) to avoid
running a shell. readpipe() (the function behind qx//) could be similarly
extended.

=head2 strcat(), strcpy(), strncat(), strncpy(), sprintf(), vsprintf()

Maybe create a utility that checks after each libperl.a creation that
none of the above (nor sprintf(), vsprintf(), or *SHUDDER* gets())
ever creep back to libperl.a.

  nm libperl.a | ./miniperl -alne '$o = $F[0] if /:$/; print "$o $F[1]" if $F[0] eq "U" && $F[1] =~ /^(?:strn?c(?:at|py)|v?sprintf|gets)$/'

Note, of course, that this will only tell whether B<your> platform
is using those naughty interfaces.

=head2 Audit the code for destruction ordering assumptions

Change 25773 notes

    /* Need to check SvMAGICAL, as during global destruction it may be that
       AvARYLEN(av) has been freed before av, and hence the SvANY() pointer
       is now part of the linked list of SV heads, rather than pointing to
       the original body.  */
    /* FIXME - audit the code for other bugs like this one.  */

adding the C<SvMAGICAL> check to

    if (AvARYLEN(av) && SvMAGICAL(AvARYLEN(av))) {
        MAGIC *mg = mg_find (AvARYLEN(av), PERL_MAGIC_arylen);

Go through the core and look for similar assumptions that SVs have particular
types, as all bets are off during global destruction.

=head2 Extend PerlIO and PerlIO::Scalar

PerlIO::Scalar doesn't know how to truncate().  Implementing this
would require extending the PerlIO vtable.

Similarly the PerlIO vtable doesn't know about formats (write()), or
about stat(), or chmod()/chown(), utime(), or flock().

(For PerlIO::Scalar it's hard to see what e.g. mode bits or ownership
would mean.)

PerlIO doesn't do directories or symlinks, either: mkdir(), rmdir(),
opendir(), closedir(), seekdir(), rewinddir(), glob(); symlink(),
readlink().

See also L</"Virtualize operating system access">.

=head2 -C on the #! line

It should be possible to make -C work correctly if found on the #! line,
given that all perl command line options are strict ASCII, and -C changes
only the interpretation of non-ASCII characters, and not for the script file
handle. To make it work needs some investigation of the ordering of function
calls during startup, and (by implication) a bit of tweaking of that order.


=head1 Tasks that need a knowledge of the interpreter

These tasks would need C knowledge, and knowledge of how the interpreter works,
or a willingness to learn.

=head2 state variable initialization in list context

Currently this is illegal:

    state ($a, $b) = foo(); 

The current Perl 6 design is that C<state ($a) = foo();> and
C<(state $a) = foo();> have different semantics, which is tricky to implement
in Perl 5 as currently the produce the same opcode trees. It would be useful
to clarify that the Perl 6 design is firm, and then implement the necessary
code in Perl 5. There are comments in C<Perl_newASSIGNOP()> that show the
code paths taken by various assignment constructions involving state variables.

=head2 Implement $value ~~ 0 .. $range

It would be nice to extend the syntax of the C<~~> operator to also
understand numeric (and maybe alphanumeric) ranges.

=head2 A does() built-in

Like ref(), only useful. It would call the C<DOES> method on objects; it
would also tell whether something can be dereferenced as an
array/hash/etc., or used as a regexp, etc.
L<http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2007-03/msg00481.html>

=head2 Tied filehandles and write() don't mix

There is no method on tied filehandles to allow them to be called back by
formats.

=head2 Attach/detach debugger from running program

The old perltodo notes "With C<gdb>, you can attach the debugger to a running
program if you pass the process ID. It would be good to do this with the Perl
debugger on a running Perl program, although I'm not sure how it would be
done." ssh and screen do this with named pipes in /tmp. Maybe we can too.

=head2 Optimize away empty destructors

Defining an empty DESTROY method might be useful (notably in
AUTOLOAD-enabled classes), but it's still a bit expensive to call. That
could probably be optimized.

=head2 LVALUE functions for lists

The old perltodo notes that lvalue functions don't work for list or hash
slices. This would be good to fix.

=head2 LVALUE functions in the debugger

The old perltodo notes that lvalue functions don't work in the debugger. This
would be good to fix.

=head2 regexp optimiser optional

The regexp optimiser is not optional. It should configurable to be, to allow
its performance to be measured, and its bugs to be easily demonstrated.

=head2 delete &function

Allow to delete functions. One can already undef them, but they're still
in the stash.

=head2 C</w> regex modifier

That flag would enable to match whole words, and also to interpolate
arrays as alternations. With it, C</P/w> would be roughly equivalent to:

    do { local $"='|'; /\b(?:P)\b/ }

See L<http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2007-01/msg00400.html>
for the discussion.

=head2 optional optimizer

Make the peephole optimizer optional. Currently it performs two tasks as
it walks the optree - genuine peephole optimisations, and necessary fixups of
ops. It would be good to find an efficient way to switch out the
optimisations whilst keeping the fixups.

=head2 You WANT *how* many

Currently contexts are void, scalar and list. split has a special mechanism in
place to pass in the number of return values wanted. It would be useful to
have a general mechanism for this, backwards compatible and little speed hit.
This would allow proposals such as short circuiting sort to be implemented
as a module on CPAN.

=head2 lexical aliases

Allow lexical aliases (maybe via the syntax C<my \$alias = \$foo>.

=head2 entersub XS vs Perl

At the moment pp_entersub is huge, and has code to deal with entering both
perl and XS subroutines. Subroutine implementations rarely change between 
perl and XS at run time, so investigate using 2 ops to enter subs (one for
XS, one for perl) and swap between if a sub is redefined.

=head2 Self ties

self ties are currently illegal because they caused too many segfaults. Maybe
the causes of these could be tracked down and self-ties on all types re-
instated.

=head2 Optimize away @_

The old perltodo notes "Look at the "reification" code in C<av.c>".

=head2 Properly Unicode safe tokeniser and pads.

The tokeniser isn't actually very UTF-8 clean. C<use utf8;> is a hack -
variable names are stored in stashes as raw bytes, without the utf-8 flag
set. The pad API only takes a C<char *> pointer, so that's all bytes too. The
tokeniser ignores the UTF-8-ness of C<PL_rsfp>, or any SVs returned from
source filters.  All this could be fixed.

=head2 The yada yada yada operators

Perl 6's Synopsis 3 says:

I<The ... operator is the "yada, yada, yada" list operator, which is used as
the body in function prototypes. It complains bitterly (by calling fail)
if it is ever executed. Variant ??? calls warn, and !!! calls die.>

Those would be nice to add to Perl 5. That could be done without new ops.

=head2 Virtualize operating system access

Implement a set of "vtables" that virtualizes operating system access
(open(), mkdir(), unlink(), readdir(), getenv(), etc.)  At the very
least these interfaces should take SVs as "name" arguments instead of
bare char pointers; probably the most flexible and extensible way
would be for the Perl-facing interfaces to accept HVs.  The system
needs to be per-operating-system and per-file-system
hookable/filterable, preferably both from XS and Perl level
(L<perlport/"Files and Filesystems"> is good reading at this point,
in fact, all of L<perlport> is.)

This has actually already been implemented (but only for Win32),
take a look at F<iperlsys.h> and F<win32/perlhost.h>.  While all Win32
variants go through a set of "vtables" for operating system access,
non-Win32 systems currently go straight for the POSIX/UNIX-style
system/library call.  Similar system as for Win32 should be
implemented for all platforms.  The existing Win32 implementation
probably does not need to survive alongside this proposed new
implementation, the approaches could be merged.

What would this give us?  One often-asked-for feature this would
enable is using Unicode for filenames, and other "names" like %ENV,
usernames, hostnames, and so forth.
(See L<perlunicode/"When Unicode Does Not Happen">.)

But this kind of virtualization would also allow for things like
virtual filesystems, virtual networks, and "sandboxes" (though as long
as dynamic loading of random object code is allowed, not very safe
sandboxes since external code of course know not of Perl's vtables).
An example of a smaller "sandbox" is that this feature can be used to
implement per-thread working directories: Win32 already does this.

See also L</"Extend PerlIO and PerlIO::Scalar">.

=head1 Big projects

Tasks that will get your name mentioned in the description of the "Highlights
of 5.12"

=head2 make ithreads more robust

Generally make ithreads more robust. See also L</iCOW>

This task is incremental - even a little bit of work on it will help, and
will be greatly appreciated.

One bit would be to write the missing code in sv.c:Perl_dirp_dup.

Fix Perl_sv_dup, et al so that threads can return objects.

=head2 iCOW

Sarathy and Arthur have a proposal for an improved Copy On Write which
specifically will be able to COW new ithreads. If this can be implemented
it would be a good thing.

=head2 (?{...}) closures in regexps

Fix (or rewrite) the implementation of the C</(?{...})/> closures.

=head2 A re-entrant regexp engine

This will allow the use of a regex from inside (?{ }), (??{ }) and
(?(?{ })|) constructs.

=head2 Add class set operations to regexp engine

Apparently these are quite useful. Anyway, Jeffery Friedl wants them.

demerphq has this on his todo list, but right at the bottom.