correction: I override 'has_subdir' in Tk::DirTree package.
Also there is some more to override in those packages for Win32 (for case-insensistive sorting, for exampe):

package ::Tk::DirTree;
sub has_subdir {1}
sub DirCmd {
    my ($w, $dir, undef) = @_;
    $dir =~ s/^(\w:)$/$1\//;
    my $h = DirHandle->new($dir) or return;
    my @names = grep( $_ ne '.' && $_ ne '..', $h->read );
    return @names;
}
sub add_to_tree {
    my( $w, $dir, $name, $parent ) = @_;

    my $image = $w->Getimage( $w->cget('-image') );
    my $mode = 'none';
    $mode = 'open' if $w->has_subdir( $dir );

    my @args = (-image => $image, -text => $name);
    if( $parent ) {             # Add in alphabetical order.
        foreach my $sib ($w->infoChildren( $parent )) {
            if( lc($sib) gt lc($dir) ) {
                push @args, (-before => $sib);
                last;
            }
        }
    }

    $w->add( $dir, @args );
    $w->setmode( $dir, $mode );
}
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