I finally got back to my development box and tried this again to produce the errors and pull some sample input, and lo, the errors are now gone

I suspect you were getting

$ perl -e'STDOUT->print'
Can't locate object method "print" via package "IO::Handle" at -e line
+ 1.
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The reason Perl can't find the method is because you didn't load IO::Handle.

You would never get that with «use IO::File;» since IO::File loads IO::Handle.