I appreciate your help, but the I still can't get it to do what I want.
I have 30 seperate files which are opened individually and from each file infomation is extracted, e.g for files 1 and 2:

file 1: H(15)
        H(16)
file 2: H(15)
        H(15)
        H(16)
[download]

Note all the information from all 30 files is stored in @hydrogen_split.

So interating over the information from file 1 should not output anything, but from file 2 H(15) should be displayed as it is repeated.
With the following code H(15) H(16) is outputed ?? Is there something wrong ?

foreach my $key (@hydrogen_split) {
$hydrogen_split{$key}++;
}

foreach my $key ( sort grep { $hydrogen_split{$key} > 1 } keys %hydrog
+en_split){
print "$key\n";
}
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Thanks a very confused Harry