=item *
+A Perl program executes only once, not once for each input line. You can
+do an implicit loop with C<-n> or C<-p>.
+
+=item *
+
The English module, loaded via
use English;
=back
-=head2 C Traps
+=head2 C/C++ Traps
-Cerebral C programmers should take note of the following:
+Cerebral C and C++ programmers should take note of the following:
=over 4
=item *
-The C<break> and C<continue> keywords from C become in
-Perl C<last> and C<next>, respectively.
-Unlike in C, these do I<not> work within a C<do { } while> construct.
+The C<break> and C<continue> keywords from C become in Perl C<last>
+and C<next>, respectively. Unlike in C, these do I<not> work within a
+C<do { } while> construct. See L<perlsyn/"Loop Control">.
=item *
-There's no switch statement. (But it's easy to build one on the fly.)
+There's no switch statement. (But it's easy to build one on the fly,
+see L<perlsyn/"Basic BLOCKs and Switch Statements">)
=item *
=item *
-Comments begin with "#", not "/*".
+Comments begin with "#", not "/*" or "//". Perl may interpret C/C++
+comments as division operators, unterminated regular expressions or
+the defined-or operator.
=item *
=item *
+A Perl program executes only once, not once for each input line. You can
+do an implicit loop with C<-n> or C<-p>.
+
+=item *
+
Backreferences in substitutions use "$" rather than "\".
=item *
The environment is not automatically made available as separate scalar
variables.
+=item *
+
+The shell's C<test> uses "=", "!=", "<" etc for string comparisons and "-eq",
+"-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which
+uses C<eq>, C<ne>, C<lt> for string comparisons, and C<==>, C<!=> C<< < >> etc
+for numeric comparisons.
+
=back
=head2 Perl Traps
# perl4 prints: {a}
# perl5 prints: 2
+=item * Parsing
+
+When perl sees C<map {> (or C<grep {>), it has to guess whether the C<{>
+starts a BLOCK or a hash reference. If it guesses wrong, it will report
+a syntax error near the C<}> and the missing (or unexpected) comma.
+
+Use unary C<+> before C<{> on a hash reference, and unary C<+> applied
+to the first thing in a BLOCK (after C<{>), for perl to guess right all
+the time. (See L<perlfunc/map>.)
+
=back
=head2 Numerical Traps
=item * Numerical
-Formatted output and significant digits
+Formatted output and significant digits. In general, Perl 5
+tries to be more precise. For example, on a Solaris Sparc:
print 7.373504 - 0, "\n";
printf "%20.18f\n", 7.373504 - 0;
# Perl4 prints:
- 7.375039999999996141
- 7.37503999999999614
+ 7.3750399999999996141
+ 7.375039999999999614
# Perl5 prints:
7.373504
- 7.37503999999999614
+ 7.375039999999999614
+
+Notice how the first result looks better in Perl 5.
+
+Your results may vary, since your floating point formatting routines
+and even floating point format may be slightly different.
=item * Numerical
Assignment of return values from numeric equality tests
does not work in perl5 when the test evaluates to false (0).
-Logical tests now return an null, instead of 0
+Logical tests now return a null, instead of 0
$p = ($test == 1);
print $p,"\n";
# perl4 prints: perl4
# perl5 prints: perl5
+=item * Regular Expression
+
+Unlike in Ruby, failed matches in Perl do not reset the match variables
+($1, $2, ..., C<$`>, ...).
=back
=item * Interpolation
-Double-quoted strings may no longer end with an unescaped $ or @.
+Double-quoted strings may no longer end with an unescaped $.
$foo = "foo$";
- $bar = "bar@";
- print "foo is $foo, bar is $bar\n";
+ print "foo is $foo\n";
- # perl4 prints: foo is foo$, bar is bar@
+ # perl4 prints: foo is foo$
# perl5 errors: Final $ should be \$ or $name
Note: perl5 DOES NOT error on the terminating @ in $bar
=item * Interpolation
-You also have to be careful about array references.
+You also have to be careful about array and hash brackets during
+interpolation.
+
+ print "$foo["
+
+ perl 4 prints: [
+ perl 5 prints: syntax error
print "$foo{"
perl 4 prints: {
perl 5 prints: syntax error
+Perl 5 is expecting to find an index or key name following the respective
+brackets, as well as an ending bracket of the appropriate type. In order
+to mimic the behavior of Perl 4, you must escape the bracket like so.
+
+ print "$foo\[";
+ print "$foo\{";
+
=item * Interpolation
Similarly, watch out for: