use strict;
use CGI;

my $q = CGI->new();

my $image = $q->param("image") or '';
if($image ne ''){
    print $q->header("image/gif");
    ## print out the rest of image here
}
else{
    print $q->header();
    print $q->start_html("This is my page!");
    ## print out HTML here...
}