So here it is with bonus, just a little longer... And now, its just a little faster than yours. Except on large identical strings where it is 3 times faster because of the first line. Without it the regexp would perform very slow.

sub comp {
    return -1 if $_[0] eq $_[1];
    local $_ = $_[0] ^ $_[1];
    /^([\x00]*)([^\x00])\2[\x00]*$/;
    $1 ? length$1 : -1;
}
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