BrowserUk,
In your example, you have created two large lists. Perhaps you could get away with just one: 
my $cnt = 0;
for (sort keys %hash) {
    $hash{$_} = ++$cnt;
}

[download]
Now, assuming that doesn't work you can take a multi-pass approach using each (which consume almost no memory). Essentially, you have two data structures designed to maintain order (pick your poison). One the first pass, you keep track of the bottom N elements leaving the 2nd data structure empty. On the second pass, you start recording the next bottom N elements while assigning the values of the first N. On the 3rd pass they swap functions. Wash, rinse, repeat.

Cheers - L~R

In reply to Re: In-place sort with order assignment by Limbic~Region
in thread In-place sort with order assignment by BrowserUk

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.