Thanks much for checking it out. You're right -- I am using Windows and the balance of my code is structurally like yours.
After posting the problem, it occurred to me that examining Text.pm might offer some clues. After all, if manual text selections are handled in the module-level code via its own 'sel' mechanism, then I should be able to do so in my program too. But I haven't done that yet. It might also be that some lower-level communication with Windows handles the manual selecting; hence the discrepancy with Linux.