One problem with your code is that you're reading the entire file into memory when you don't need to. Here's an alternative that doesn't (untested):

my $data_file = "ActiveItems2.txt";
open(DAT, $data_file) || die("Could not open file: $!");
my %seen;
while (<DAT>) {
    chomp;
    $seen{$_}++;
}
close(DAT);

for (sort keys %seen){
    print "$_\n";
}
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