You can use mod_rewrite as suggested by maa; see also URL Rewriting Guide. If you want to use mod_perl, see Using mod_perl to rewrite URLs. If you want to roll your own, you can do something like this inside your user.cgi:

#!/usr/bin/perl

use CGI;
use URI;

$q = CGI->new;
$user = $q->param('user');

$uri = URI->new;
$uri->scheme('http');
$uri->host('www.mywebsite.com');
$uri->path("/users/$user");

print $q->redirect( -uri => $uri->as_string );

exit;
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