The fileevent method arguments look suspect. Those do not compare favorably with the examples I found in the Tk::Event POD. That recommends a structure that in this instance would look more like:

$mw->fileevent( *\LOG, 'readable' => \&insert_txt )
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Which is close to what you have as => and , are interchangeable (although the arrow is semantically easier to follow in this case). You may need to pass a reference to the filehandle instead of the filehandle, but I don't have a good handle on filehandles, so I'd recommend further inquiry after trying something more like the above. And most importantly your callback appears to be wrapped in an anonymous list reference, when all you really need is a sub reference.