my %hash;
my @tests = (
    [ 'string1', 'string2', 'string3', 'string4' ],
    [ 'string3', 'string4'  ],
    [ 'string1', 'string2', 'string3' ],
);
for my $t (@tests) {
   my $hr = \%hash;
   my @strings = @$t;
   increment($hr, @strings);
}

sub increment {
    my ($hr, @strings) = @_;
    if (@strings == 1) {
        # simple case
        ++$$hr{$strings[0]};
    }
    else {
        # Peel the first string from the list
        my $first_string = shift @strings;
        # Get the associated hash reference
        $hr = $$hr{$first_string};
        # Call ourself...
        increment($hr, @strings);
    }
}