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"Any two-argument open(), readpipe() (aka qx//) and similar operators found within the lexical scope of this pragma"
I haven't dived into the guts (well, I have dived into guts related to open.pm but not recently and not in relation to this specific case), but it appears that the only thing within the lexical scope of the pragma is the passing of a file handle to an XS module. That XS module reads from the handle and the reading from the handle triggers "magic" (as you put it) that causes a file to be opened.
The opening is not done by code within the lexical scope of the pragma. Perhaps the documentation should say that it impacts 'open' within the temporal scope of the pragma? I doubt it actually does that, though (that wouldn't match my memory of the guts the last time I dived into them).
But if it isn't temporal scope, then I'm hard pressed to explain how it could actually work in this case. Perhaps somebody will explain it. I don't plan to spend time investigating this particular mystery.
I doubt the original poster's expressed desire for ignorance will lead to success when dealing with UTF-8 streams. Unfortunately, UTF-8 was defined in a way and supported by Unix (and Perl) in ways that make handling it correctly very often require significant diving into a lot of details.
In reply to Re^3: Why Doesn't Text::CSV_XS Print Valid UTF-8 Text When Used With the open Pragma? ("lexical")