If a subroutine is defined after it is called, the call has to use parenthesis.

Not quite. If a subroutine is declared after it is called, the call has to use parenthesis. You can always define it afterwards.

>perl -wle"sub foo; foo; sub foo { print 'Hi!' }"
Hi!
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Granted, it's very rare for a subroutine to be declared somewhere other than when it's being defined.