use 5.16.3;
use warnings;
use autodie;
open my $in,  "<", "numbers.txt";
open my $out, ">", "sum.txt";
while (<$in>) {
    m{\S}    or  next; # skip empty lines
    m{^\s*#} and next; # skip comment lines
    m{^[-.0-9+*/ ]+$} or die "Line too dangerous to evaluate!"; # . for floats
    # pattern needs extending for math (sqrt, log, sin, cos, ...) support and exponentials with e notation
    # and 'x' for hexadecimal, '(', and ')' for grouping etc etc. Anyway, it is dangerous.
    # I'd not use eval for safety reasons, but if your validator is strict enough
    my $result = eval $_;
    $@ and die "Invalid expression on line $.";
    print $out $result;
    }
close $in;
close $out;