$gLast_id=$tree->insert($gLast_id, "end",  -text => "some_text" );
$tree->item($gLast_id, -open => "true");
#$tree->focus($gLast_id);
__END__
Can't call method "insert" on an undefined value at - line 1.
[download]

Sorry you had trouble...try this complete mini-program

 
    use Tkx; my $mw = Tkx::widget->new("."); #std line
    
    $frm_main= $mw->new_frame();   #parent widg probably a frame outsi
+de of this compound control 
    $frm_main->g_pack( -expand => 0, -fill => 'x',  -side => top); 
    
    my  $tree=$frm_main->new_ttk__treeview; #http://www.tkdocs.com/tut
+orial/tree.html
    $tree->g_pack( -expand => 0, -fill => 'x',  -side => top); 
    
    $gLast_id="root";
    $gLast_id=$tree->insert("", "end", -id => $gLast_id, -text => "roo
+t");
    $tree->item($gLast_id, -open => "true");
    #$tree->focus($gLast_id);
    
    $tree->g_bind(  "<Insert>",  sub {
        $gLast_id=$tree->insert($gLast_id, "end",  -text => "some_text
+" );  
        $tree->item($gLast_id, -open => "true"); #http://www.tkdocs.co
+m/tutorial/tree.html
        #$tree->focus($gLast_id);
    });
    
    Tkx::MainLoop();
[download]

Your program works for me, if the tree control has focus, and I hit insert, then the newly added item has focus,  $tree->focus($gLast_id) worked

You can give the widget focus with  $tree->g_focus;

Does that help?

I found the solution....it was

$tree->selection_set($gLast_id);
[download]

not focus

yes selection is different from focus :)