Not just because the import list can change... The more important reason for doing the ->import()
step every time is that a Perl program can be spread across many files (modules or otherwise) and those files can define many namespaces, each of which may need to import symbols from the same module.
If ->import() was only called the first time, then only the first namespace to use the module would import its symbols and any other namespaces which expected them to be available would fail. And fail in very-difficult-to-debug ways, since the failure would be dependent on which files are loaded and in what order.