Of course eval() would work, but it's slow, insecure and boring. I would probably do it with a table of operations stored in a hash:

#!/usr/bin/perl -w
use strict;
my %ops = ( '+' => sub { $_[0] + $_[1] },
            '-' => sub { $_[0] - $_[1] },
            '*' => sub { $_[0] * $_[1] },
            '/' => sub { $_[0] / $_[1] } );
print $ops{$ARGV[1]}->($ARGV[0], $ARGV[2]), "\n";
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Of course, if you want to support more than just a single binary operation you'll need to parse the expression. For that I'd use Parse::RecDescent. In fact - I did! You can check out HTML::Template::Expr for a rather elaborate example of this technique.

-sam