It is very common for various implementation quirks to be used with regard to loops, in the very-important name of efficiency.

Rather than efficiency, I think reliability (or perhaps one should better say coherence) is the key concern. If a topicalized (i.e., localized and aliased) Perl-style loop iterator were left un-de-localized upon exit from the loop, to what would it be aliased? An arbitrary element of some named or referenced array? An item in a temporary list, perhaps a literal (i.e., something unwritable), or the (writeble) return value of a function call? (This point has been touched upon in other replies.) Such a state of affairs seems like a recipe for some very perplexing bugs.

IOW, if not de-localized, exactly what is the nature of the thing to which  $_ would remain aliased after loop exit in this code:

c:\@Work\Perl\monks>perl -wMstrict -le
"sub F { return 4; }
 sub G { return 5; }
 sub H { return 6; }
 ;;
 for (F(), G(), H()) { ++$_;  printf qq{$_ }; }
"
5 6 7
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And why would one want to do that?