Might as well toss this in, then:

...
my $op = shift or die "You must specify an operation followed by numbe
+rs.\n";
###########################################################
die "At least two numbers required.\n" unless @ARGV > 1;
###########################################################
die "Non-numeric input.\n" if grep /\D/, @ARGV;
...
[download]

Despite jwkrahn's warning, I'd want to implement an "eval" wrapped around the operation, too - after carefully vetting the data. It's quite nice for those "Divide by zero" errors.

-- 
Human history becomes more and more a race between education and catastrophe. -- HG Wells