use strict; use warnings;

 my $msg = 'this is a test';
 my $pat = '';
 my $rep = '';
 $msg =~ s/$pat/$rep/;
 print $msg, "\n";

##</code><code>##

 this is a test

##</code><code>##

 use strict; use warnings;

 my $c = 'this is another test';
 $c =~ s/^this is/XX/;

 my $msg = 'this is a test';
 my $pat = '';
 my $rep = '';
 $msg =~ s/$pat/$rep/;
 print $msg, "\n";

##</code><code>##

 a test

##</code><code>##

perl -v

This is perl, v5.8.8 built for x86_64-linux-thread-multi-ld