my %{"platform_$site"};
${"platform_$site"}{"thingy"}= 5;
foreach my $x ( keys %{"platform_$site"}) {
  ...

##</code><code>##

my %platform;
$platform{$site}{"thingy"}= 5;
foreach my $x ( keys %{$platform{$site}}) {
  ...

##</code><code>##

push @{$platform{$site}}, 1,2;
$platform{$site}[5]= 5;
foreach my $x ( @{$platform{$site}}) {
  ..