Here's a quickie: this came up on perl5-porters yesterday: someone filed what they thought was a bug, but was bitten by the fact that a capture that does not match returns undef, rather than the empty string. One way around it is to just switch of warnings when substituting values containing undef:

use strict;
use warnings;
use Test::More tests => 6;

is(munge('R'),   'text',  'R');
is(munge('R1'),  '1text', 'R1');
is(munge('R2'),  '2text', 'R2');
is(munge('R3'),  'text3', 'R3'); # this here is the trickiness
is(munge('R13'), 'R13',   'R13 unchanged');
is(munge('R4'),  'R4',    'R4 unchanged');

sub munge {
    my $str = shift;
    no warnings;
    $str =~ s/^R(?:([12])|(3))?$/${1}text$2/;
    $str;
}
[download]

Another alternative, even more icky in my books, is to say:

sub munge {
    my $str = shift;
    $str =~ s{^R(?:([12])|(3))?$}
             {(defined $1 ? $1 : '') . 'text' . (defined $2 ? $2 : '')
+}e;
    $str;
}
[download]

So I ask the monks... is there an elegant (clean, simple) way to do the substition without /e or no warnings? Note that the real tokens R, 1, 2... in real life are meta characters, so substr is ruled out, it must be a regexp, and so one just needs to make sure that $1 and $2 are always strings, empty or otherwise.