http://www.perlmonks.org?node_id=406242


in reply to Gratuitous use of Perl Prototypes

sub PUSH {
perlsub say:
Subroutines whose names are in all upper case are reserved to the Perl core, as are modules whose names are in all lower case. A subroutine in all capitals is a loosely-held convention meaning it will be called indirectly by the run-time system itself, usually due to a triggered event. Subroutines that do special, pre-defined things include AUTOLOAD, CLONE, DESTROY plus all functions mentioned in perltie and PerlIO::via.