Woot I think I got it. Check it out (I also added comments)

use warnings;
use strict;

my $i = $#ARGV;                             #Scalar for array length

if ($ARGV[0] eq "add") {                    #Checks for add
    my $rtn = add(@ARGV[1..$i]);     #Calls add subroutine
    print "The sum is: $rtn";
}

elsif ($ARGV[0] eq "multiply"){                    #Checks for multipl
+y
    my $rtn = multiply(@ARGV[1..$i]);       #Calls multiply subroutine
    print "Their product is: $rtn";
}

sub add {                            #add subroutine
    my ($sum);
    $sum=0;
    foreach $_(@_) {                 #Loops through the array
        $sum +=$_;                   #Adds each array element to the r
+est
    }
    return $sum;                     #Returns the sum value
}

sub multiply {                       #multiply subroutine
    my $product=1;                   
    foreach $_(@_) {                 #Loops through the array
        $product*=$_;                #Multiplys each element to last
    }
    return $product;                 #Returns the product value
}
[download]

How does that look?