Since you don’t seem to care to check for errors, you had better put this at the top of your program:

use 5.10.1;
use autodie;
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Alternately, check your opens:

open(F, "<", $filename)
    || die "can't open $filename: $!";
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I also suggest you get used to using parentheses around your function calls to make them easier to read and parse, although that isn’t really the problem here.