Can you please specify what output you expect from the given sample files and why? You code does not output anything. To make it ouput something, I had to add the key among its values in hash1:

k_aX,k_aX
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It then prints all the keys from hash2. That is probably not what you wanted.

UPDATE:
BTW, my ($hash1, $hash2)= @ARGV or die "$!"; will return strange errors (or none at all) if no arguments are supplied.

@ARGV == 2 or die "Specify two arguments.\n";
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should work better. Also, using double quotes in readhash("$hash1") where $hash1 is a file name is useless. Just readhash($hash1);.