We hope these notes will save you from confusion and lost
sleep when writing Perl scripts on VMS. If you find we've
missed something you think should appear here, please don't
-hesitate to drop a line to vmsperl@genetics.upenn.edu.
+hesitate to drop a line to vmsperl@newman.upenn.edu.
=head1 Installation
=item %ENV
-Reading the elements of the %ENV array returns the
-translation of the logical name specified by the key,
-according to the normal search order of access modes and
-logical name tables. If you append a semicolon to the
-logical name, followed by an integer, that integer is
-used as the translation index for the logical name,
-so that you can look up successive values for search
-list logical names. For instance, if you say
+The operation of the C<%ENV> array depends on the translation
+of the logical name F<PERL_ENV_TABLES>. If defined, it should
+be a search list, each element of which specifies a location
+for C<%ENV> elements. If you tell Perl to read or set the
+element C<$ENV{>I<name>C<}>, then Perl uses the translations of
+F<PERL_ENV_TABLES> as follows:
+
+=over 4
+
+=item CRTL_ENV
+
+This string tells Perl to consult the CRTL's internal C<environ>
+array of key-value pairs, using I<name> as the key. In most cases,
+this contains only a few keys, but if Perl was invoked via the C
+C<exec[lv]e()> function, as is the case for CGI processing by some
+HTTP servers, then the C<environ> array may have been populated by
+the calling program.
+
+=item CLISYM_[LOCAL]
+
+A string beginning with C<CLISYM_>tells Perl to consult the CLI's
+symbol tables, using I<name> as the name of the symbol. When reading
+an element of C<%ENV>, the local symbol table is scanned first, followed
+by the global symbol table.. The characters following C<CLISYM_> are
+significant when an element of C<%ENV> is set or deleted: if the
+complete string is C<CLISYM_LOCAL>, the change is made in the local
+symbol table, otherwise the global symbol table is changed.
+
+=item Any other string
+
+If an element of F<PERL_ENV_TABLES> translates to any other string,
+that string is used as the name of a logical name table, which is
+consulted using I<name> as the logical name. The normal search
+order of access modes is used.
+
+=back
+
+F<PERL_ENV_TABLES> is translated once when Perl starts up; any changes
+you make while Perl is running do not affect the behavior of C<%ENV>.
+If F<PERL_ENV_TABLES> is not defined, then Perl defaults to consulting
+first the logical name tables specified by F<LNM$FILE_DEV>, and then
+the CRTL C<environ> array.
+
+In all operations on %ENV, the key string is treated as if it
+were entirely uppercase, regardless of the case actually
+specified in the Perl expression.
+
+When an element of C<%ENV> is read, the locations to which
+F<PERL_ENV_TABLES> points are checked in order, and the value
+obtained from the first successful lookup is returned. If the
+name of the C<%ENV> element contains a semi-colon, it and
+any characters after it are removed. These are ignored when
+the CRTL C<environ> array or a CLI symbol table is consulted.
+However, the name is looked up in a logical name table, the
+suffix after the semi-colon is treated as the translation index
+to be used for the lookup. This lets you look up successive values
+for search list logical names. For instance, if you say
$ Define STORY once,upon,a,time,there,was
$ perl -e "for ($i = 0; $i <= 6; $i++) " -
_$ -e "{ print $ENV{'story;'.$i},' '}"
-Perl will print C<ONCE UPON A TIME THERE WAS>.
-
-The key C<default> returns the current default device
-and directory specification, regardless of whether
-there is a logical name DEFAULT defined. If you try to
-read an element of %ENV for which there is no corresponding
-logical name, and for which no corresponding CLI symbol
-exists (this is to identify "blocking" symbols only; to
-manipulate CLI symbols, see L<VMS::DCLSym>) then the key
-will be looked up in the CRTL-local environment array, and
-the corresponding value, if any returned. This lets you
-get at C-specific keys like C<home>, C<path>,C<term>, and
-C<user>, as well as other keys which may have been passed
-directly into the C-specific array if Perl was called from
-another C program using the version of execve() or execle()
-present in recent revisions of the DECCRTL.
-
-Setting an element of %ENV defines a supervisor-mode logical
-name in the process logical name table. C<Undef>ing or
-C<delete>ing an element of %ENV deletes the equivalent user-
-mode or supervisor-mode logical name from the process logical
-name table. If you use C<undef>, the %ENV element remains
-empty. If you use C<delete>, another attempt is made at
-logical name translation after the deletion, so an inner-mode
-logical name or a name in another logical name table will
-replace the logical name just deleted. It is not possible
-at present to define a search list logical name via %ENV.
-It is also not possible to delete an element from the
-C-local environ array.
+Perl will print C<ONCE UPON A TIME THERE WAS>, assuming, of course,
+that F<PERL_ENV_TABLES> is set up so that the logical name C<story>
+is found, rather than a CLI symbol or CRTL C<environ> element with
+the same name.
+
+When an element of C<%ENV> is set to a defined string, the
+corresponding definition is made in the location to which the
+first translation of F<PERL_ENV_TABLES> points. If this causes a
+logical name to be created, it is defined in supervisor mode.
+(The same is done if an existing logical name was defined in
+executive or kernel mode; an existing user or supervisor mode
+logical name is reset to the new value.) If the value is an empty
+string, the logical name's translation is defined as a single NUL
+(ASCII 00) character, since a logical name cannot translate to a
+zero-length string. (This restriction does not apply to CLI symbols
+or CRTL C<environ> values; they are set to the empty string.)
+An element of the CRTL C<environ> array can be set only if your
+copy of Perl knows about the CRTL's C<setenv()> function. (This is
+present only in some versions of the DECCRTL; check C<$Config{d_setenv}>
+to see whether your copy of Perl was built with a CRTL that has this
+function.)
+
+When an element of C<%ENV> is set to C<undef>,
+the element is looked up as if it were being read, and if it is
+found, it is deleted. (An item "deleted" from the CRTL C<environ>
+array is set to the empty string; this can only be done if your
+copy of Perl knows about the CRTL C<setenv()> function.) Using
+C<delete> to remove an element from C<%ENV> has a similar effect,
+but after the element is deleted, another attempt is made to
+look up the element, so an inner-mode logical name or a name in
+another location will replace the logical name just deleted.
+In either case, only the first value found searching PERL_ENV_TABLES
+is altered. It is not possible at present to define a search list
+logical name via %ENV.
+
+The element C<$ENV{DEFAULT}> is special: when read, it returns
+Perl's current default device and directory, and when set, it
+resets them, regardless of the definition of F<PERL_ENV_TABLES>.
+It cannot be cleared or deleted; attempts to do so are silently
+ignored.
Note that if you want to pass on any elements of the
C-local environ array to a subprocess which isn't
logical names are read, in order to fully populate %ENV.
Subsequent iterations will not reread logical names, so they
won't be as slow, but they also won't reflect any changes
-to logical name tables caused by other programs. The C<each>
-operator is special: it returns each element I<already> in
-%ENV, but doesn't go out and look for more. Therefore, if
-you've previously used C<keys> or C<values>, you'll see all
-the logical names visible to your process, and if not, you'll
-see only the names you've looked up so far. (This is a
-consequence of the way C<each> is implemented now, and it
-may change in the future, so it wouldn't be a good idea
-to rely on it too much.)
-
-In all operations on %ENV, the key string is treated as if it
-were entirely uppercase, regardless of the case actually
-specified in the Perl expression.
+to logical name tables caused by other programs.
+
+You do need to be careful with the logicals representing process-permanent
+files, such as C<SYS$INPUT> and C<SYS$OUTPUT>. The translations for these
+logicals are prepended with a two-byte binary value (0x1B 0x00) that needs to be
+stripped off if you want to use it. (In previous versions of perl it wasn't
+possible to get the values of these logicals, as the null byte acted as an
+end-of-string marker)
=item $!