Aha, your problem there is that you are treating a scalar has reference as a hash, however they are two distinct things. What you need to be doing is dereferencing your hash reference when using it e.g

foreach (keys %$enthalpy_hash) {
  if(exists $hash->{$_}) {
    push @common, "$_ = ", $enthalpy_hash->{$_} * $hash->{$_}, "\n";
    push @total, $enthalpy_hash->{$_} * $hash->{$_}, "\n";
  }
}
[download]

See. tye's References quick reference and the venerable perlref for more info on references and dereferencing in perl. On a side note the likes of strict will catch this sort of error upon a simple perl -c.
HTH

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broquaint