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.