use strict;      # always!

my $op = shift;

if ($op =~ /add/) {
	print "The sum is: ", add(@ARGV), "\n";
}
elsif ($op =~ /multiply/) {
	print "The product is: ", multiply(@ARGV), "\n";
}

##</code><code>##

sub add {
   my $sum = 0;
   $sum += $_ for @_;
   return $sum;
}

# and if that's too consise - try this one:
sub add {
   my $sum = 0;
   foreach my $number (@_) {
      $sum = $sum + $number;
   }
   return $sum;
}