What's the difference between return; and return undef; in Perl

Question

Is there a difference between a subroutine that does

return;

and one that does?

return undef;

Answer 1

return; will return an empty list in list context but undef in scalar context. return undef; will always return a single value undef even in list context.

In general, it's usually not a good idea to return undef; from a subroutine normally used in list context:

sub foo { return undef }
if ( my @x = foo() ) {
    print "oops, we think we got a result";
}

In general, it's usually not a good idea to return; from a subroutine normally used in scalar context, because it won't behave as the user expects in list context:

sub foo { return }
%x = ( 'foo' => foo(), 'bar' => 'baz' );
if ( ! exists $x{'bar'} ) {
    print "oops, bar became a value, not a key";
}

Both of these errors happen quite a bit in practice, the latter more so, perhaps because subs that are expected to return a scalar are more common. And if it's expected to return a scalar, it had better return a scalar.