Perlで，URI->newに空文字列を渡すと，URIのクラスがいい感じになって，URI::_genericというクラスが返る．HTTPだと思って$uri->hostとか呼んでると，URI::_genericにはhostメソッドとかないので死ぬ．
第二引数に'http'とか書くとURI::httpが返ってくるのでhostとか呼べる．
$ URI->new('')->host
Can't locate object method "host" via package "URI::_generic" at (eval) line 1.
default
$ URI->new('', 'http')->host
[
[0] undef
]
The $scheme argument is only used when $str is a relative URI. It can be either a simple string that denotes the scheme, a string containing an absolute URI reference, or an absolute URI object. If no $scheme is specified for a relative URI $str, then $str is simply treated as a generic URI (no scheme-specific methods available).
URI - Uniform Resource Identifiers (absolute and relative) - metacpan.org - Perl programming language
