Looking at the source, it looks as though there technically is no default hostname. Check out the _connect sub, and you'll see that it checks whether hostname is defined to decide whether to try a TCP socket. But looking a few lines up at new, there isn't any default for hostname, it has to be passed as an argument to new():

sub new {
    my ($class, %args) = @_;

    my $self = bless {
        hostname              => $args{hostname},
        path                  => $args{path}     || DEFAULT_UNIX_SOCKE
+T,
        port                  => $args{port}     || DEFAULT_PORT_NUMBE
+R,
#...snip!
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