Personally, I avoid mocking modules, because they add one more layer of indirection, and mocking up a single function isn't too hard:

use Some::Module;
use Test::More tests => 2;

{
my $some_function_called;
local *some_function = sub {
    $some_function_called++;
    return @canned_results;
};

my @res = other_function_that_calls_some_function();
is $some_function_called, 1, 'We asked for resource existence';
is_deeply \@res, ['expected', 'results'], 'We got good results';
}
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Of course, if you need to mock out large object functionality, one of the Mock modules might help you better, but I like to replace just the relevant parts of an object and have come to the conclusion that I best write my constructors and methods in a way such that I can pass in all "needed" objects or data as optional parameters. If they are not present, they get taken from elsewhere, but otherwise the passed-in values get used.