This is tangentially discussed in How can I remove duplicate elements from a list or array?, a perlfaq. You can construct your hash, and output whenever you hit a duplicate:

my %seen;
while (my $line = <PFILE>) {
   @ID = split(/:/, $line);
   my $id = $ID[0];
   if ($seen{lc($id)}++) {
       print "'$id' is duplicate\n";
   }
}
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Note you had incorrect syntax for accessing a list element. I would also recommend against using variables with the same character sequence but different cases for names.