my $type= $q->param('type');
$type ||= 'display';

if( 'display' eq $type ) {
    # output Content-Type: text/html
    # Output HTML page
} elsif( 'download' eq $type ) {
    # output Content-Type: application/x-download
    # Output file
} else {
    # Should never get here unless somebody has misconfigured the script or is playing around with the parameters
    # output Content-Type: text/html
    # Output HTML page
};