Apache ErrorDocument directive issues an internal redirection filling your %ENV with special fields identified by REDIRECT_ prefix containing useful informations. Besides that, you can use the usual CGI methods inside your script to retrieve passed parameters. Here is a little snippet that will show the values available to your "error handler":

#!/usr/bin/perl

use CGI;
use Data::Dumper;

my $q = CGI->new;
%v = $q->Vars;

print $q->header;
print "<pre>\n";
print Dumper(\%v), "\n";
print Dumper(\%ENV), "\n";
print "</pre>";
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Please note that you don't need to pass the original error code (ie 404, etc), because that code is already available via REDIRECT_STATUS.

Ciao, Valerio