$ perl -le " sub foo { } "

$ perl -le " sub foo spaces { } "
Illegal declaration of subroutine main::foo at -e line 1.

$ perl -le " use diagnostics; sub foo spaces { } "
Illegal declaration of subroutine main::foo at -e line 1 (#1)
    (F) A subroutine was not declared correctly.  See perlsub.

Uncaught exception from user code:
        Illegal declaration of subroutine main::foo at -e line 1.