use strict;
use warnings;

my $match = "ABCDABCDXBCDABCXABCDXXCDABXD";
my $str   = "ABCDABCDABCDABCDABCDABCDABCDABCDABCDABCD";

$match =~ s/x/[abcd]/ig;

print "Matched $1\n" if $str =~ /($match)/i;

##</code><code>##

Matched ABCDABCDABCDABCDABCDABCDABCD