@ISA will not load the modules, you have to do that yourself. @ISA merely tells Perl what namespaces to look through to find methods. If the namespace is empty (because you haven't loaded a module), Perl doesn't particularly mind.

The example I gave you showed you how you would call a method as a class method. The equivalent object-style call was in the comments. Speedwise, I think it's a slight benefit to call things as class methods, because Perl doesn't have to search the inheritance tree.

Using your original example, your code might looks like this:

use Data::Dumper;
use User;

my $dbh = User->new();

use Company;
# Pretend that $dbh is a Company and call its do_something method,
# equivalent to $dbh->do_something, if $dbh were a Company
$dbh->Company::do_something;
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