use CGI; 
(store parameters etc); 
print $cgi->redirect('the.other.host/url-with-parameters');
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In that case, you could accept the request in your script, record the parameters, then do a LWP::Simple request in the background, and send the results of that request to the client. Note that in that case, the other server will see ALL the requests as coming from your server's IP - which may not be what you want.

Update:Right, as b10m pointed out, LWP::Simple doesn't do POST, only get (the fact that it accepts the URL as the single parameter should have clued me in). Yes, you'd have to use LWP::UserAgent, and it's post method.

Thanks very much matija - I shall check out LWP::UserAgent.

Maybe my brain has been damaged by lack of caffiene, but it seems to me the other monks are answering a different question than the one you asked.

Current Situation

You have a web page which displays a form. When the user completes the form, it is sent to a CGI script, which transmits the information to a third party.

Of course, other possibilities exist; for example, that your web site displays a page containing a form, but when the Submit button is pressed, the data goes tot he third party, without ever coming to your site.

Desired Change

You wish the data to be saved at your system.

Trivial Solution

Is it possible to configure Apache so that the POST arguments are made part of the log? Is it possible the information already is being logged?

Less Trivial Solution

When the button is pressed, a CGI script is invoked. Modify the script so it saves the data before sending it off to the third party.

Somewhat Tougher Solution

If the data is going directly to the third party, modify the web page containing the form, so it invokes your CGI script instead of someone else's. Your script can save the data, then re-direct the user to the remote script, or it can invoke the remote script "secretly" and then pass the return values to the user.

use HTTP::Request::Common;
  $ua = LWP::UserAgent->new;
  $ua->request(POST 'http://somewhere/foo', [foo => bar, bar => foo]);
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collects some information from the user and transmits it to a third party.

Sends it to a third party, how? Have you tried the redirect method of CGI?

Here's what you want, assuming that the 3rd party is allowing the data to be sent as a GET request:

#!c:/perl/bin/perl -w
$|++;

use strict;
use CGI::Simple;
use URI::Escape;

my $CGI = CGI::Simple->new();

# save your copy of the information here
# using whatever method you prefer


# now we redirect the user to the 3rd party with the data
my $query = join(
   ';', map {
      uri_escape($_) . '=' . uri_escape($CGI->param($_))
   } $CGI->param()
);

print $CGI->redirect(
   'http://their.server.com/foo/bar.pl?' . $query
);
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In spirit of permitting multiple values to the same parameter name, change the query builder I provided to this new one:

my $query = join(';',
   map {
      my $k = shift @$_;
      join(';', map { uri_escape($k) . '=' . uri_escape($_) } @$_)
   }
   map { [$_, $CGI->param($_)] } $CGI->param()
);
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You could also do:

my $query = join(';',
   map {
      my $k = $_;
      join(';', map {
         uri_escape($k) . '=' . uri_escape($_)
      } $CGI->param($_))
   } $CGI->param()
);
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I'm thinking along the same lines as TomDLux .... unless I'm missing something, couldn't you write the info you send to the third party to a (log) file?

Typically (in my situations), someone "submits" from a form, and the output is emailed to the third party, and the event (and any info needed) is written to a file...

in the script, there's something like:

open(MAIL,"|$mailprog -t"); 
.
.
print MAIL $q->param('WHATEVER'),"\n";
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open (LOGFILE,">>$logfile") || die "NFG $logfile!\n";
.
.
.
print LOGFILE $q->param('WHATEVER')."\n";
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Hell, if your emailing the output to the third party, you could BCC yourself ... but again, you don't mention how you're sending the info to the thrid party, so it's tough to tell if the solution could be *that* siumple.

Maybe I'm missing something ?? , but this appears to be question simple enough that even *I* have offered a response ;)