That particular warning comes about because this statement:

push @TEMP, $cur_amt, $past_amt;
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towards the end of sub formatC2 is called with $past_amt undefined. You can fix this by providing $past_amt with a default value near the start of the subroutine:

$past_amt = '';
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I tracked this down by looking at the contents of @OUTPUT after the first while loop finishes. As fishmonger says, you can use Data::Dumper or Data::Dump for this. I prefer Data::Dump:

use Data::Dump;

...

while (<$fh>) {
        chomp;  #Removes the control character
        my @LINES = split(/\n/, $_, -1); #Split on line break creating
+ array
        my @RECORDS = formatC2(@LINES);
        $OUTPUT[$seq_num] = [@RECORDS];    
        last;
}
close $fh;

dd \@OUTPUT;
exit(0);
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The last ensures that I get only the first iteration of the while loop, and the exit helps me to see what I need to see (in this case, the contents of @OUTPUT) by omitting everything that comes after.

Hope that helps,