Then you have problems with your input, cause I tested my update before submitting it.

%a = (
  A => 1,
);

print 'A' if not exists $a{'A'};  # prints nothing
print 'B' if not exists $a{'B'};  # prints B
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Try use warnings; to see if you're trying to print undef, and like pg suggested, make sure the // actually matched. You could also give us a sample of IN and IN2 which doesn't provide the expected output.