I made a similar modification, but based it on LWP's support for proxies defined in environment variables.

if ( defined $ENV{HTTP_proxy} )
{
  $ua->env_proxy();
}
[download]

And then after the HTTP::Request gets initialized:

my $proxy_user = $ENV{HTTP_proxy_user};
my $proxy_pass = $ENV{HTTP_proxy_pass};
if ( defined $proxy_user && defined $proxy_pass )
{
  $req->proxy_authorization_basic("$proxy_user", "$proxy_pass");
}
[download]

Hope this helps! --Glenn