You might need an if() block that handles your cases and you might need a while loop if you want to sanitize input... Here's some concept code:

#!/usr/bin/perl

use strict;
use warnings;

print "What would you like to do? <add|subtract|multiply|divide>: ";
chomp(my $todo = <STDIN>);

if($todo eq "add") {
    print "$first_num + $second_num = ", $first_num + $second_num, "\n
+";
} elsif($todo eq "subtract") {
    print "$first_num - $second_num = ", $first_num - $second_num, "\n
+";
}
...
[download]

And so on.