The problem is that you never actually hand your HTML to HTML::Mason to process - you are just attaching a string literal. While I've never tried to use Mason like this, Using Mason from a standalone script in HTML::Mason::Interp gives this example:

 
    my $outbuf;
    my $interp = new HTML::Mason::Interp (comp_root=>'<component root>
+',
                                      data_dir=>'<data directory>',
                                          out_method=>\$outbuf);
    my $retval = $interp->exec('<component path>', <args>...);
    open(F,">mason.out");
    print F $outbuf;
    close(F);
    print "return value of component was: $retval\n";
[download]

so you'll want to include some similar processing on your $html variable.