the file you try to read is empty.

Update: Your code can be shortened a lot.

my $sum;
                                                                      
+ 
open(FILE, "<$filename") || die "file $filename can't open : $!";
while(<FILE>) { $sum += ( split /\s+/ )[37]; }
close FILE;
print $sum;
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or even simpler as a one liner:

shell> perl -naF\s+ -e "$sum+=$F[2]; END{print $sum}" file
[download]