my ($frg) = @ARGV;
$frgfile = "$frg";
open(frgfile) or die("Unable to open FRG file");
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That's very confusing form. It's a good practice to always start your scripts with

use strict;
use warnings;
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this may help you to avoid many problems. In this case you would see some warnings. I'd write this as follows:

open my $fd, '<', $ARGV[0] or die $!;
while (<$fd>) {
...
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Note how you can store filehandle in scalar variable.