gistfile1.pl

Furl::HTTP->new->request(
    method => 'GET',
    host   => 'example.com',
    port   => 80,
    path   => '/',
);
# User-Agent: Furl::HTTP/0.32

Furl::HTTP->new(agent => 'gfx')->request(
    method => 'GET',
    host   => 'example.com',
    port   => 80,
    path   => '/',
);
# User-Agent: gfx

Furl::HTTP->new(headers => ['User-Agent' => 'yappo'])->request(
    method => 'GET',
    host   => 'example.com',
    port   => 80,
    path   => '/',
);
# User-Agent: Furl::HTTP/0.32
# User-Agent: yappo

Furl::HTTP->new(agent => 'Foo')->request(
    method  => 'GET',
    host    => 'example.com',
    port    => 80,
    path    => '/',
    request => ['User-Agent' => 'Hoge'],
);
# USer-Agent: Foo
# User-Agent: Hoge