Your regexes are correct. (assuming formElement:definition|Alternative definition is the input.)

What is incorrect is that you named your own defined subroutine: split. It is a built-in function.

Your own subroutine will only be called when you call it via &split($something). Else the built-in one will be called.

Here are two suggestions:
a) but a warn/print statement (for debugging purpose) in your function and see if it is called.
b) enable warnings.