push @{$dataPos{$data[$_]}}, $_ for 0 .. $#data;

##</code><code>##

my $foo;        # $foo is undefined
push @$foo, 23;
print "I found @{$foo}\n";

##</code><code>##

push @{ undef() }, 99;

##</code><code>##

my $foo = undef;
shift @{$foo};
print "Foo: $foo\n";  # prints Foo: ARRAY(0x12121212)