thor

Your problem here is that your Foo module has already exported &foo into Bar's namespace by the time you have mocked anything. What you want to do is to mock Foo before loading Bar, like this:

use strict;
use warnings;

use Test::MockModule;

my $mock = Test::MockModule->new('Foo');
$mock->mock('foo', sub {print "Successfully mocked!\n";});

require Bar;
my $bar = Bar->new();
$bar->bar();
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Personally I don't think this is ideal since you need to require Bar rather than use it.

However, since you are exporting &Foo::foo into Bar, you could ignore the fact &Foo::foo came from Foo, and just mock &foo inside Bar. Like this:

use strict;
use warnings;

use Test::MockModule;
use Bar;

my $mock = Test::MockModule->new('Bar');
$mock->mock('foo', sub {print "Successfully mocked!\n";});
     
my $bar = Bar->new();
$bar->bar();
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This too is probably not ideal.

Of course, if you simply do not export &Foo::foo, and instead call with the fully qualitied packag name (Foo::foo()) then you original test code will work.