This should fix it. You need to tell the mark where it is to be set. Here is the changed part.

$text->insert('0.0', "First\n");
$text->insert('end', "Second\n");
# $text->markSet('Foo', '3.0'); #make mark at start of line 3
$text->markSet('Foo', 'end-1 chars'); #make mark after last inserted t
+ext
$text->markGravity('Foo','left');
$text->insert('end', "Third\n");
$text->insert('Foo', "-inserted-\n");
MainLoop;
[download]

HTH, --traveler