Here is a code snippet from a Perl/Tk password generating app I wrote that copies stuff to the clipboard. Hope it helps.

my($entry) = $main_frame->Entry ( -textvariable => \$password );
$entry->clipboardClear();
$entry->clipboardAppend($password);
$entry->SelectionClear();
$entry->SelectionHandle( sub { $password } );
$entry->SelectionOwn;
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