Thank you Neil - I will make sure not to put so much in next time (I was afraid if I didn't, and I had made an error somewhere further up, it would have been missed). Didn't mean to overcrowd!

Please excuse my ignorance, but I have been trying to load both files into their own hashes, the two hashes are:

%max_unchanged
%max_changed
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Maybe there is a much simpler way.. but if I use this:

my @calc;
$diff;
foreach my $key (keys %max_changed) {
my $diff = $max_unchanged{$key} - $max_changed{$key};
push (@calc, $diff);
}

print @calc, "\n";
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I still get the same error.. :-s...

Re your second point, I wasn't too sure what you meant by

"Your open statements should use variables, not bare words (depreciated), and use an operator."

When you say "bare words" are you referring to an FILEHANDLE being called FILE?
Many thanks, E