Good question.

First a quick note: Although %hash->{key} works you shouldn't do that. $hash{key} is the right way to use a hash. This is a known bug.

Anyhow, the problem boils down to that   $hash{KEY} is interpreted as   $hash{'KEY'} and that a bareword to the left of => also is interpreted as a string. So what you have is:

  my %hash1 = ('TESTVALUE', 'OK');
  my %hash2 = (123,       , 1);
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What you want to have is   my %hash1 = (TESTVALUE() => 'OK'); or   my %hash1 = (TESTVALUE, 'OK'); When getting the value with the constant you need to make it a non-bareword:   $hash1{+TESTVALUE} or   $hash1{TESTVALUE()} Update:
I intentionally avoided mentioning & as a way of disambiguate the constant. This is because it will confuse and mislead people to think that you can do &foo; instead of foo(), which isn't the case. See perlfaq7.

Hope I've helped,
ihb