At some point you are saying your regex fails when you do:

$x =~ /$y/;
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yes? That being the case, then do this for debugging:

my $match = $x =~ /$y/;
printf "x=%s, y=%s\n", $x, $y unless $match;
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So this will print out something every time they don't match, which is the case you are looking for, right?