my %foo;
my $bar;

sub foo : lvalue
{
    my ($key) = @_;

    if (some_complex_condition())
    {
         do_some_stuff($foo{a});
         $key = "b";
    }
    if (other_complex_condition())
    {
         $foo{a} .= do_some_other_stuff($foo{b},$foo{c});
         $key = "a";
    }

    defined($key)?
       exists($foo{$key})?
            $foo{$key}
            : $foo{$key} = undef
       : $bar;
}