Proxy method does not work using LWP::UserAgent for HTTPS request

I have following sample getting file via HTTPS protocal using
LWP::UserAgent module.

#!/usr/bin/perl

use LWP::UserAgent;
use LWP::Debug qw(+);

$uri = $ARGV[0];
$ua = LWP::UserAgent->new;
$ua->timeout(10);
#$ENV{HTTPS_PROXY} = 'myproxy.mydomain:8080'; # (A) this works
$ua->proxy('https', 'myproxy.mydomain:8080'); # (B) this
don't works at all

$response = $ua->get($uri);
if ($response->is_success) {
print $response->decoded_content;
}
else {
die $response->status_line;
}

Setting HTTPS_PROXY environment variable works fine, but
use $ua->proxy setting proxy don't work. It gaives following trace.

koroke:~/tmp$ https_get.pl https://www.somedomain.jp/
LWP::UserAgent::new: ()
LWP::UserAgent::proxy: https myproxy.mydomain:8080
LWP::UserAgent::request: ()
LWP::UserAgent::send_request: GET https://www.somedomain.jp/
LWP::UserAgent::_need_proxy: Proxied to myproxy.mydomain:8080
LWP::UserAgent::request: Simple response: Not Implemented
501 Protocol scheme '' is not supported at https_get.pl line 17.

Who can explain that's different?
Thanks.
.