string1,string2,string3,string4
string3,string4
string1,string2,string3
##</code><code>##

%hash{'string1'}{'string2'}{'string3'}{'string4'}++;
%hash{'string3'}{'string4'}++;
%hash{'string1'}{'string2'}{'string3'}++;
##</code><code>##

foreach my $k1 (keys %hash){
  foreach my $k2 (keys %hash{$k1}){
...
  }
}