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use strict;
use warnings;
...
}

untie %H;

|10.1353/ham.2005.0020| does not exist!
|10.1353/ham.2005.0020| eq |10.1353/ham.2005.0020|

while(my $k = each %H) {
   print "$k does not exist\n"  if (!exists $H{$k});
}