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#!/usr/bin/perl -w
use strict;
...
linux-2.4.3.tar
linux-2.10.6.tar
linux-2.10.50.tar

a1.5
a1.5b
...
linux-2.4.29a.tar
linux-2.10.6.tar
linux-2.10.50.tar

        $key =~ s[(?<!\.)(\d+)(\.\d+)(?!\.)][
            pack( "N", length($1) ) . $1 . $2 ]ge;
        $key =~ s[(\d+)][
            pack( "N", length($1) ) . $1 ]ge;