What you want here is a hash.
Basically, what you do is iterate through your list of strings, assigning each one as a key to your hash. The value of each key will be the number of times that particular string has been "seen". Here is a very simple example to demonstrate what I mean:

#!/usr/bin/perl -w
use strict;
use Data::Dumper::Simple;

my %strings;

while (<DATA>) {
    chomp;
    $strings{$_}++;
}

print Dumper(%strings);

__DATA__
string1
string2
string3
string1
string1
string7
string2
[download]

Which prints..

%strings = (
             'string3' => 1,
             'string7' => 1,
             'string1' => 3,
             'string2' => 2
           );
[download]

Have a look in the Q&A section under hashes for some more examples.

Hope this helps,
Darren :)