I'd use a Hash of Arrayrefs.

use Modern::Perl;
use Data::Dump qw /dump/;
my %hash;

while (<DATA>) {
    chomp;
    my ($item, $sequence, $value) = m/(aixFsMountPoint|aixFsSize|aixFs
+Free)\.(\d+)\s=\s"?([^"]*)/;
    next unless $item;
    $hash{$item}[$sequence] = $value;
}

say dump(\%hash);

__DATA__

actual STDOUT:

aixFsMountPoint.1 = "/"
aixFsMountPoint.2 = "/usr"
aixFsMountPoint.3 = "/var"
aixFsMountPoint.4 = "/tmp"
aixFsMountPoint.5 = "/local.home"
aixFsMountPoint.6 = "/admin"
aixFsMountPoint.7 = "/proc"
aixFsMountPoint.8 = "/opt"
aixFsMountPoint.9 = "/var/adm/ras/livedump"
aixFsMountPoint.10 = "/net"
aixFsMountPoint.11 = "/net/nim01at/opt/CBC-CFG"
aixFsMountPoint.12 = "/net/nim01at/opt/CBC-CFG"
aixFsSize.1 = 3072
aixFsSize.2 = 8512
aixFsSize.3 = 10240
aixFsSize.4 = 4096
aixFsSize.5 = 64
aixFsSize.6 = 128
aixFsSize.7 = 0
aixFsSize.8 = 4096
aixFsSize.9 = 3072
aixFsSize.10 = 0
aixFsSize.11 = 7168
aixFsSize.12 = 7168
aixFsFree.1 = 2631
aixFsFree.2 = 3232
aixFsFree.3 = 2373
aixFsFree.4 = 1923
aixFsFree.5 = 46
aixFsFree.6 = 127
aixFsFree.7 = 0
aixFsFree.8 = 3000
aixFsFree.9 = 3071
aixFsFree.10 = 0
aixFsFree.11 = 3463
aixFsFree.12 = 3463
[download]

Output

{
  aixFsFree       => [undef, 2631, 3232, 2373, 1923, 46, 127, 0, 3000,
+ 3071, 0, 3463, 3463],
  aixFsMountPoint => [
                       undef,
                       "/",
                       "/usr",
                       "/var",
                       "/tmp",
                       "/local.home",
                       "/admin",
                       "/proc",
                       "/opt",
                       "/var/adm/ras/livedump",
                       "/net",
                       "/net/nim01at/opt/CBC-CFG",
                       "/net/nim01at/opt/CBC-CFG",
                     ],
  aixFsSize       => [undef, 3072, 8512, 10240, 4096, 64, 128, 0, 4096
+, 3072, 0, 7168, 7168],
}
[download]

Of course, whether such datastructure is useful or not will ultimately depend on what you want to do with the data.