You need to require or use the modules you're inheriting from, so that Perl has access to it's code.

package Project::Client::Mode;
require Project::Client; # <---
@ISA = ('Project::Client');
use strict;

...
[download]

You might also want to check out the Inheritance section of perltoot, and base (which will combine the require and @ISA-population for you in one easy step).

    --k.