use List::Util 'reduce';
my $op = shift @ARGV;
if ($op =~ /add/) {
    my $sum= reduce { $a + $b } @ARGV;
    print "The sum is $sum\n";
}
elsif ($op =~ /mult/) {
    my $product= reduce { $a * $b } @ARGV;
    print "The product is $product\n";
} else {
    print "Huh? I don't know how to '$op'?\n";
}