"here I tryed to make a sum. with "my $n= "$val+8";", so that I can obtain a vlue like -31.9999400000 (instead of '-39.9999400000'+8). But unfortunately that does not work. So how can I solve this problem? (that was the reason why I thought it is a string value)."

I know you've only been using Perl for a short amount of time, but I don't think you've really got an understanding of the basics. Try reading "perlintro -- a brief introduction and overview of Perl": you need a solid foundation on which to build further knowledge.

With regard to your "$val+8" question, this code should explain why it's not working:

#!/usr/bin/env perl -l

use strict;
use warnings;

my @items = (-39.9999400000);
my $val = $items[-1];
print $val;

my $n = "$val+8";
print $n;

my $x = $val+8;
print $x;
[download]

Output:

-39.99994
-39.99994+8
-31.99994
[download]