The following code works fine:
use WWW::Mechanize qw(!update_html);

$w = new WWW::Mechanize;
$w->get("file:hello.txt");
$html = $w->content;
$html =~ s/Hello World/Goodbye Everyone/isg;
$w->update_html( $html );
print $w->content;

it picks up a file containing the worlds "Hello World" and prints "Goodbye Everyone"

If I change the WWW::Mechanize to WWW::Scripter, thus:
use WWW::Scripter qw(!update_html);

$w = new WWW::Scripter;
$w->get("file:hello.txt");
$html = $w->content;
$html =~ s/Hello World/Goodbye Everyone/isg;
$w->update_html( $html );
print $w->content;

The result is, "Hello World" is printed. Hence I'm assuming the update_html in WWW::Scripter doesn't function incorrectly.

Thanks for your interest.