Much better to use strictures (use strict; use warnings;), lexical file handles and three parameter open:

use strict;
use warnings;

open my $inFile, '<', $fn or die "Failed to open $fn: $!";
open my $outFile, '>', $fo or die "Failed to open $fo: $!";
while (<$inFile>) {
    ...
    print $outFile "...";
}

close $inFile;
close $outFile;
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Oh, and don't slurp files (use while (<$inFile>) instead).

Update: fixed typo in code - thanks wfsp