The sketch offered by pvaldes makes a good starting point... but there are a lot of possible cases about which you've given us inadequate information.

For rs.txt where the format is similar to any of these, one (long-winded but explicit) solution might be:

#!/usr/bin/perl
use strict;
use warnings;
use 5.016;

my @line = ("1+2 blah blah",
            "leading words 1+2 blah blah",
            "spaces around arith op: 1 + 2 blah blah",
            );
for my $line(@line) {
    chomp $line;           
    my ($foo,$bar) = split /\s*\+\s*/, $line;
    $foo =~ s/.*([0-9]+).*/$1/;
    $bar =~ s/.*([0-9]+).*/$1/;
    say $foo + $bar . " is total at Ln 16";
}

__END__

output:

3 is total at Ln 16
3 is total at Ln 16
3 is total at Ln 16
[download]

But, clearly, this is not a comprehensive list of possible data formats. Next time, please do as FAQS, such as How do I post a question effectively?, On asking for help and Writeup Formatting Tips suggest.