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sub compare {
  (($_[0] ^ $_[1]) =~ tr/\0//c) < 2;
}

print((compare('abc', 'abcd') ? 'ok' : 'not ok'), "\n");

sub compare {
   length $_[0] ==  length $_[1] && (($_[0] ^ $_[1]) =~ tr/\0//c) < 2;
}