Replace the Xs with a character class match [ABCD] then use the resulting string as the match string:

use strict;
use warnings;

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

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

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

Matched ABCDABCDABCDABCDABCDABCDABCD
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