use File::Basename;

my $db = $cgi->url_param('db');
my $datei = $cgi->url_param('datei');

download("/fester_pfad/".$db."/".$datei);

sub download {
my $file = shift;
my $fn = fileparse($file);
print $cgi->header(-type=>'application/octet-stream',-attachment=>$fn);
}