use warnings;
use strict;

my $user_defined_string = "abcabcabc";
my $user_defined_search = '(a)';

##</code><code>##

my $user_defined_replace = '---$1---';

##</code><code>##

my $user_defined_replace = '"---".$1."---"';

print "before: $user_defined_string\n";
$user_defined_string =~ s/$user_defined_search/$user_defined_replace/ee;
print "after: $user_defined_string\n";

##</code><code>##

before: abcabcabc
after: ---a---bcabcabc