use List::Util qw( reduce );

my %ops = (
    add => sub { reduce { $a + $b } @_ },
    multiply => sub { reduce { $a * $b } @_ },
    # etc ...
);

my $op = shift or die "You must specify an operation followed by numbers.\n";
die "Non-numeric input.\n" if grep /\D/, @ARGV;
die "Invalid operation.\n" unless exists $ops{$op};

print "$op (@ARGV)= ". $ops->(@ARGV) . "\n";