You can read the files in tandem:

while (defined(my $min = <MIN>) && defined(my $max = <MAX>)) { ... }
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Take a look at open. The first two examples there show (the preferred) 3-argument form and also use $! for a more meaningful error message if something goes wrong.

Add use strict; and use warnings; to the top of your code to get messages about problems that may exist. And, as you indicate you're new to Perl, also adding use diagnostics; will provide more verbose messages.