Good job asking for feedback on your code. To elaborate on what jwkrahn said about the range operator, the range operator is for *generating* a list of numbers, but what you want to do is iterate over an existing array.

So the range operator lets you say:

    1..5

which is equivalent to the list:

    1, 2, 3, 4, 5

So if, say, your ARGV array in the original example was ('add', 1, 7, 22, 5) then $ARGV[1]..$ARGV[$i] would evaluate to 1..5, ie: (1, 2, 3, 4, 5). Echoing oko1's comment about a dispatch table being the way to go, here is how I would write your script:

use warnings;
use strict;

my %dispatch = (
    add => sub { my $sum = 0; $sum += $_ for @_; $sum; },
    multiply => sub { my $prod = 1; $prod *= $_ for @_; $prod; },
);

my $op = shift @ARGV;

print $dispatch{$op}->(@ARGV);
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