#!perl -l

@strings = <DATA>;

for(@strings)
{
	chomp;
	s/,/'}{'/g;
	s/^/\$hash{'/;
	s/$/'}++/;
	print "eval $_";
	eval $_;
}

use Data::Dumper;
print Dumper \%hash;

__DATA__
string1,string2,string3,string4
string3,string4
string1,string2,string3

##</code><code>##

c:\>test.pl
eval $hash{'string1'}{'string2'}{'string3'}{'string4'}++
eval $hash{'string3'}{'string4'}++
eval $hash{'string1'}{'string2'}{'string3'}++
$VAR1 = {
          'string3' => {
                         'string4' => 1
                       },
          'string1' => {
                         'string2' => {
                                        'string3' => 5393161
                                      }
                       }
        };