package MyStuff;
use HTML::Template;
use strict;
use warnings;
use CGI();

sub MyStuff::RunCGI {
    my $q = CGI->new;
    my( $headers, $body ) = MyStuff::DispatchCGI( $q );
    print $headers, $body;
}
sub MyStuff::DispatchCGI {
    my( $q ) = @_;
    my $page = $q->page || 'default';
    $page = 'ajax' if $q->request_method eq 'POST' and ...;
    return ThisPage( $q ) if $page eq 'thisone';
    return ThatPage( $q ) if $page eq 'thatone';
    return AjaxPage( $q ) if $page eq 'ajax';
}


sub MyStuff::ThisPage {
    my( $q ) = @_;
    my $template = HTML::Template->new ...;
    $template->param( foo => scalar $q->param('foo') );
    ...
    ...
    return $q->headers, $template->output;
}

sub MyStuff::AjaxPage {
    my( $q ) = @_;
    ...
    return $headers, $body;
}