If you use CGI, you can utilize the 'redirect' method:

use strict;
use CGI;

my $CGI = new CGI;

if (my $file = $CGI->param('file')) {
    print $CGI->redirect('http://localhost/'.$file);
}
else {
    print $CGI->header;
    print "stuff";
}
[download]

but I would not trust the user - make sure you untaint that file name first.

Jeff

R-R-R--R-R-R--R-R-R--R-R-R--R-R-R--
L-L--L-L--L-L--L-L--L-L--L-L--L-L--