Let's say file1.txt contains integers.

5
-10
15.4
20

The following markup will get the sum of the integers in file1.txt. Running this will script will produce 30.4, which is the sum.

my $file = "/path/to/file.txt";
open(FH, "<", $file) or die "cannot open $file $! \n";

my $sum = 0;
while (my $line = <FH>) {
  $sum += $line;
}

close FH;

print "$sum\n";