There's no documentation on the choice of internal storage format, because it's an internal detail.
 ...except when it affects the documented behavior of functions like lc in certain situations.
Neither operators nor subs can return arrays, just scalars.
 I don't follow your meaning here. In what sense is this not an array being returned: 
@words = split(/ /, 'This is a sentence.');

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Returning strings from a sub should have no effect on their storage format.
 ...yet sometimes does, per Re^7: Seeking Perl docs about how UTF8 flag propagates.

In reply to Re^2: Seeking Perl docs about how UTF8 flag propagates by raygun
in thread Seeking Perl docs about how UTF8 flag propagates by raygun

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