my %foo;
get_set( 'foo' ) = 42;
print get_set( 'foo' );

sub get_set :lvalue {
    my $key = shift;
    print "key is $key\n";
    my $junk;
    $key eq 'foo' ? $foo{$key} : $junk;
}
__END__
key is foo
key is foo
42