#!/usr/bin/perl -w
use strict;
my $lhs = "abc";
my ($string1, $string2,$string3, $string4);
$string1 = "qrsabcwty";
$string2 = "qrsabcwty";
$string3 = "qrsabcwty";
$string4 = "qrsabcwty";

my %lhsCompiled;
#lets say the regex we want is

$string1 =~  s/($lhs)/XXX$1/;

# Then I compile it.
my $rhsCompiled = qr{XXX$1};
$lhsCompiled{$lhs} = qr{($lhs)};

# This regex will not work

$string2 =~ s/$lhsCompiled{$lhs}/$rhsCompiled/;
# The left hand side works, but the right had side comes
#  out as (?-xism:XXXabc)
# it returns qrs(?-xism:XXXabc)wty

# instead I make the right hand side into a code block
#  using the e modifier which identifies the right hand side
# as code block or a subroutine call.
sub lhsSub {return "XXX".$1}
$string3 =~ s/$lhsCompiled{$lhs}/lhsSub/e;
$string4 =~ s/$lhsCompiled{$lhs}/"XXX".$1/e;

# I suspect using a subroutine is faster.  It will be compiled,
#  I am not certain if the code will be compiled
print "string1 $string1\n";
print "string2 $string2\n";
print "string3 $string3\n";
print "string4 $string4\n";

#The results are
# string1 qrsXXXabcwty
# string2 qrs(?-xism:XXXabc)wty
# string3 qrsXXXabcwty
# string4 qrsXXXabcwty