Your question is addressed exactly by Class::DBI. It is particularly good at translating database relationships into object methods, so your users and projects question all but disappears. Assuming you have a Class::DBI class for each of user and project, put this in the My::User class:

__PACKAGE__->hasa_list(
    'My::Project', 
    ['name_of_field_in_project_table'],
    'projects',
);
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And then to retrieve the projects all you have to say is:

my $user = My::User->retrieve($id);
my @projects = $user->projects();
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I use it all the time and i can't recommend it highly enough.