use strict;
use warnings;

my %c;
$_ = join '', <DATA>;

$c{lc($1)}++ while m/([a-zA-Z]+(?:'[a-zA-Z]+)?)/g;
s/([a-zA-Z]+(?:'[a-zA-Z]+)?)/$1 . ($c{lc($1)} > 1 ? "[$c{lc($1)}]" : '')/eg;

print;

__DATA__
I need to know how to compare items within a string... I have dropped a textfile into an array, but now I need to check whether words in that text are repeated throughout. I have split the text; as I only want the text to be manipulated. Maybe it's better to split it like this; So anyway, I basically need to check now across my string whether any elements in my string are repeated, and if so, how many times. I've read alot about manipulating arrays, but they're all based on arrays that you create yourself, rather than arrays created by opening a textfile, so I'm not sure how to manipulate my array. Any help would be much appreciated.

##</code><code>##

use strict;
use warnings;

my (%c, @w);
$_ = lc join '', <DATA>;

while (m/([a-z]+(?:'[a-z]+)?)/g) {
    push @w, $1 if !$c{$1}++;
}
for (@w) {
    print "$_ : $c{$_}\n" if $c{$_} > 1;
}