when I try to check $! and $^E, I get 'Illegal seek'.

The problem with checking those error variables is that they are set by low-level operations, not the high level ones you are using.

For example: The "Illegal Seek" message can mean that a repetitive Next File or similar operation has nothing more to return. The error makes sense in the context of the low-level seek operation, but not in the higher level Find All, where it simply means: You've got them all.

Context is everything, and unless the module author has done a very thorough job, they will often make no sense in terms of the higher level code you are using.