HTML doc generation is controlled by the following lines of ActivePerl::PPM::Client,

    my $install_html = eval { require ActivePerl::DocTools; };
    $install_html = $args{install_html} if defined $args{install_html}
+;
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Unfortunately, it's called by ActivePerl::PPM::GUI as follows:

            $ppm->install(
                area => $INSTALL_AREA,
                packages => \@install_pkgs,
                run_cb => \&run_cmd,
            );
[download]

Fix:

            $ppm->install(
                area => $INSTALL_AREA,
                packages => \@install_pkgs,
                run_cb => \&run_cmd,
                ( exists($ENV{INSTALL_HTML})
                    ? ( install_html => $ENV{INSTALL_HTML} )
                    : ()
                ),
            );
[download]

That could be patched fairly easy.

perl -i.bak -ple"s/\$ppm->install\(\K/exists(\$ENV{INSTALL_HTML}) ? ( 
+install_html => \$ENV{INSTALL_HTML} ) : ()," c:\progs\perl5100\lib\Ac
+tivePerl\PPM\GUI.pm
[download]