You can to analyze error message 
use LWP::UserAgent;
$ua = new LWP::UserAgent;
$ua->proxy('http',"http://".$proxy);
$ua->timeout(5);
$req = new HTTP::Request POST => 'http://any.com/cgi-bin/any.pl';
$req->content('name=value');
$res = $ua->request($req);

$err_msg=$res->error_as_HTML();
if($err_msg =~ m/500 read timeout/)
{
   #set longer timeout and retry submition or jump to next proxy
}
else
{...

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In reply to Re: form submission with LWP::UA using proxies by Nightblade
in thread form submission with LWP::UA using proxies by m3LLow

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