because only one of them does something different than FUNC()

Actually, all of them do something different as shown below:

> perl -le'@_=2;sub a($$) {shift or 1};print &a'
2
> perl -le'@_=2;sub a($$) {shift or 1};print &a()'
1
> perl -le'@_=2;sub a($$) {shift or 1};print a()'
Not enough arguments for main::a at -e line 1, at end of line
Execution of -e aborted due to compilation errors.
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Sorry for the pedantry.