Ah. It appears that everyone (including me) missed this. 

#!/usr/bin/perl -w
use strict;

my @list= <DATA>;
my @sorted= @list[
    map { unpack "N", substr($_,-4) }
    sort
    map {
        my $key= $list[$_];
        $key =~ s[((?<!\.)(\d+)\.\d+(?!\.)|\d+)][
            my $len= length( defined($2) ? $2 : $1 );
            pack( "N", $len ) . $1 . ' ';
        ]ge;
        $key . pack "N", $_
    } 0..$#list
];
print @sorted;
__END__
a1.5
a1.5b
a1.55
a1.55b
a1.6
a1.6b
linux-2.4.28.tar
linux-2.4.29.tar
linux-2.4.29a.tar
linux-2.4.3.tar
linux-2.10.6.tar
linux-2.10.50.tar

[download]

Produces 

a1.5
a1.5b
a1.55
a1.55b
a1.6
a1.6b
linux-2.4.3.tar
linux-2.4.28.tar
linux-2.4.29.tar
linux-2.4.29a.tar
linux-2.10.6.tar
linux-2.10.50.tar

[download]

Update: Third-time lucky. The original regexes are below in HTML comments.

- tye        

In reply to Re^3: Alpha number sort (two truer to spec) by tye
in thread Alpha number sort by gam3

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.