You could say I'm a bit familiar with small/embedded/IoT type stuff, but I admit I completely glazed over that statement in your OP and missed it entirely.

Can you provide the specs of the particular board you're using and the OS information? Perl will run in tight memory constraints, but it's the storage space that could be an issue. How does the "Onion" attach to disk? Is it custom like a Pi, or is it hard like an Arduino?

Perhaps with that knowledge, it may be possible to pack up the modules you need, then do a single untar or something...