smarter ?? faster - for that you'll have to benchmark...

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11138432
use warnings;

sub randomstring { join '', map +(qw(a b))[rand 2], 1 .. shift }

for (1 .. 10)
  {
  match( $_, randomstring(4), randomstring(4), 1);
  }

sub match
  {
  my ($n, $x, $y, $m) = @_;
  $x =~ /(.+)$(??{
    ($1 ^ substr $y, 0, length $1) =~ tr!\0!!c <= $m ? '' : '(*FAIL)'
    })/ and print "$n:($-[1],@{[ length $1 ]})($x,$y)\n";
  }
[download]

Outputs:

1:(1,3)(abba,baaa)
2:(3,1)(bbbb,aaba)
3:(1,3)(abba,abab)
4:(1,3)(abbb,bbaa)
5:(2,2)(baba,bbbb)
6:(1,3)(bbab,aabb)
7:(0,4)(aaaa,aaab)
8:(2,2)(baba,baab)
9:(2,2)(aaba,baaa)
10:(1,3)(bbab,bbba)
[download]

I hope I understood your problem correctly...

Update: fixed $_ to $n