As someone mentioned, the easiest/shortest way is with a proxy; e.g., Plack::App::Proxy–

That could be wrapped up in a proper script to control URIs and capturing data and what gets proxied and what doesn't, etc.

Because I was curious, I tried whipping up a Plack::Request/HTTP::Response based version. Turns out it's quite easy. Though it seems to work, this code is a simplistic first stab and I have no idea how fit it is for your, or any, actual task.

The code is shown after the usage… I admit it's confusing, but it was more fun/terse to have the code display itself.

Run it

I stripped some headers (Host/Agent/Accept) from the output to save space.

plackup /home/moo/post-echo.psgi -r
Watching /home/moo/lib /home/moo/post-echo.psgi for file updates.
HTTP::Server::PSGI: Accepting connections at http://0:5000/
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Use it

moo@cow[2258]~>curl http://0:5000/
OHAI GET

moo@cow[2259]~>curl http://0:5000/ -X HEAD
OHAI HEAD

moo@cow[2260]~>curl http://0:5000/ -X POST
POST http://example.org/
User-Agent: ...snip...

moo@cow[2261]~>curl http://0:5000/ -d "internet=cats in tubes"
POST http://example.org/
Content-Length: 22
Content-Type: application/x-www-form-urlencoded

internet=cats in tubes
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Have it echo its own code :P

Demonstrate that file uploads (apparently) work fine too.

curl http://0:5000/ -F code=@/home/moo/post-echo.psgi
POST http://example.org/
Expect: 100-continue
Content-Length: 2408
Content-Type: multipart/form-data; boundary=--------------------------
+--2db0025e93ba

------------------------------2db0025e93ba
Content-Disposition: form-data; name="code"; filename="post-echo.psgi"
Content-Type: application/octet-stream

#!/usr/bin/env perl
# filename: post-echo.psgi
use strictures;
use Plack::Request;
use HTTP::Request;

sub {
    my $env = shift;
    my $request = Plack::Request->new($env);

    my $response = $request->new_response(200);
    $response->content_type("text/plain; charset=UTF-8");
    if ( $request->method eq "POST" )
    {
        $request->input->read( my $buffer, $request->header("Content-L
+ength"), 0 );
        my $rerequest = HTTP::Request
            ->new( "POST", "http://example.org/" , $request->headers, 
+$buffer );

        $response->body( $rerequest->as_string );
    }
    else
    {
        $response->body("OHAI " . $request->method . "\n");
    }
    $response->finalize;
};
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