sub Foo {print "Foo!\n"}         # named subroutine / non-closure
my $foo= sub { print "Bar!\n"};  # anonymous / non-closure
my $bar= sub { $foo->() };       # anonymous / closure
sub FooBar { $bar->() }          # named subroutine / closure

sub takes_a_callback {
  my $callback=shift;
  $callback->();
}

takes_a_callback($_) foreach \&Foo,$foo,$bar,\&FooBar;