A simple search here for natural sort turns up lots of techniques. Here is a rewrite of my favorite using the "one true sort" (fast, flexible, stable sort).

#!/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+)][ pack "N", $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]

Producing

a1.5
a1.5b
a1.6
a1.6b
a1.55
a1.55b
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]