undef isn't the same as "nothing":

#!/usr/bin/perl -w
use strict;

sub ret_undef   { return undef }
sub ret_nothing { return; }

print ret_nothing(), "\n";
print ret_undef(), "\n";

my @foo = ret_nothing();
print "Elements in foo: ", scalar @foo, "\n";
print @foo;

my @bar = ret_undef();
print "Elements in bar: ", scalar @bar, "\n";
print @bar;
[download]

Are you sure? Consider:

use strict;
use warnings;
use Data::Dump::Streamer;

my $value = undef;

print !!$value;

my $nothing = nothing ();
my $retNothing = retNothing ();
my $retUndef = retUndef ();
my $retFalse = retFalse ();
my $retFalse2 = -retFalse ();

Dump (\$nothing);
Dump (\$retNothing);
Dump (\$retUndef);
Dump (\$retFalse);
Dump (\$retFalse2);


sub nothing {
}

sub retNothing {
    return;
}

sub retUndef {
    return undef;
}

sub retFalse {
    return 1 == 0;
}
[download]

Prints:

$SCALAR1 = \do { my $v = undef };
$SCALAR1 = \do { my $v = undef };
$SCALAR1 = \do { my $v = undef };
$SCALAR1 = \do { my $v = '' };
$SCALAR1 = \do { my $v = 0 };
[download]

Updated to add two false cases

and in list context :)

...

my @nothing = nothing ();
my @retNothing = retNothing ();
my @retUndef = retUndef ();
my @retFalse = retFalse ();
my @retFalse2 = -retFalse ();

Dump (\@nothing);
Dump (\@retNothing);
Dump (\@retUndef);
Dump (\@retFalse);
Dump (\@retFalse2);

...
[download]

Prints:

$ARRAY1 = [];
$ARRAY1 = [];
$ARRAY1 = [ undef ];
$ARRAY1 = [ '' ];
$ARRAY1 = [ 0 ];
[download]

---

DWIM is Perl's answer to Gödel

Try it in list context.