I would say that if you have a module explicitly calling functions in the main package, your module is not modular at all! If the "global" function is needed everywhere, it should be in a separate module, importable by the other modules. Or if a module needs another package (let's say the main package) to provide it with a function, perhaps it would be cleaner if main sent a reference instead:

package main;

Bar::bar(\&foo);

sub foo {
  # do stuff
}

package Bar;

sub bar {
  # need to call a subroutine provided by the caller
  my $subref = shift;
  $subref->();
}
[download]

The only uses I have had for &sub are to generate subroutine references and to override subroutine prototypes.