sub a{ print 'from a'; undef $_ };;
sub b{ print 'from b'; undef $_ };;

@r = ( \&a, \&b );;

$_->( 1 ) for @r;;
from a
from b

$_->( 1 ) for @r;;
Use of uninitialized value in subroutine entry at (eval 13)
Undefined subroutine &main:: called at (eval 13)