Just first-post trolling. In little time, there will be better answers, probably using optimization techniques like the famous Schwartzian Transform.

print sort {
    my ($aa) = $a =~ /(\d+)\s*$/;
    my ($bb) = $b =~ /(\d+)\s*$/;
    $bb <=> $aa;

} <DATA>;

__DATA__
 10.47.147.0   Free: 63    In-use: 99    P_InUse: 61
 10.47.147.0   Free: 63    In-use: 99    P_InUse: 65
 10.47.147.0   Free: 63    In-use: 99    P_InUse: 70
 10.47.147.0   Free: 63    In-use: 99    P_InUse: 69
 10.47.147.0   Free: 63    In-use: 99    P_InUse: 63
 10.47.147.0   Free: 63    In-use: 99    P_InUse: 66
[download]

Have fun

2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$
[download]