If you want the original text, marked up to show words that appear more than once:

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 th
+at text are repeated throughout. I have split the text; as I only wan
+t 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 a
+ny elements in my string are repeated, and if so, how many times. I'v
+e read alot about manipulating arrays, but they're all based on array
+s that you create yourself, rather than arrays created by opening a t
+extfile, so I'm not sure how to manipulate my array. Any help would b
+e much appreciated.
[download]

If you just want a list of all the words that appear more than once, in order of first appearance:

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;
}
[download]