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

[download]
And why would one want to do that?

Give a man a fish:  <%-{-{-{-<

In reply to Re^2: loop surprise by AnomalousMonk
in thread loop surprise by morgon

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.