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.

Answer 2

Given

sub foo { return; } sub bar { return undef; } 

In scalar context, they behave the same.

my $foo = foo();   # $foo is undef my $bar = bar();   # $bar is undef 

In list context, they behave differently

my @foo = foo();   # @foo is ()  (an empty list) my @bar = bar();   # @bar is ( undef )  (a one-element list) 

Note that a one-element list is a true value in boolean context, even though the only element is undef.

In general, it's usually not a good idea to return undef; from a subroutine, because of how it behaves in context.