The line:

while(chomp($line=<F>)) {

has a bug. The chomp function does not return the string which has been chomped; it returns the number of characters removed. So, if the last line in the file does not end in a newline character, chomp returns zero, the while loop terminates, and the last line of data is lost. If the last line does end in a newline character, the last line is processed but the loop doesn’t terminate, so on the next iteration chomp is called on a value of undef, which results in a warning if use warnings; is in effect. The correct approach is:

while ($line = <F>) {
    chomp $line;
    ...
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BTW, you do always begin your scripts with:

use strict;
use warnings;
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don’t you?

Hope that helps,