Looks like you've found (yet another :) bug in the Windows port of Perl/Tk. At least, I'm assuming you're running under Windows, because the following works fine under Linux:

#!/usr/bin/perl
use Tk; use Tk::ROText;
my $mw = MainWindow->new();
my $rot = $mw->ROText;
$rot->pack;
$rot->insert('end','foo ');
$rot->insert('end','the bar',['mytag','sel']);
$rot->insert('end',' baz');
MainLoop;
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Of course, the same code does nothing under my Win32 box, so I'd assume that is the problem you are referring to. Unfortunatly, I could find no way around it -- normally $mw->idletasks() or $mw->update() fixes problems such as these, but had no effect here. I'll poke a bit more; but failing that, I don't see a simple answer. BTW, this problem is not limited to Tk::ROText -- it appears to be a problem even in Tk::Text. I'm amazed that nobody has noticed this before.
Of course, if you're running under Linux, then disregard all of the above, and take the above code as a working example. :)