...
use List::MoreUtils qw(uniq);

open($fh, '<', 'filetoparse.txt') or die "error $!";
    chomp(my @lines = <$fh>);
close $fh;

my @old_ids;
foreach my $line (@lines) {
    @data = split(/\t/, $line);
    push @old_ids, $data[12];
}  

my @uniq_ids = uniq (@old_ids); #get unique ids
    
#step through unique ids for new id using old ids
for my $i (0..$#uniq_ids) {
    
    print $uniq_ids[$i] * 1;  #line 78 of my code to test for a number
    
    my $stmt = "SELECT * FROM temp_topics WHERE old_id = ?";
    my $topic = $dbh->selectrow_hashref($stmt, undef, $uniq_id[$i]);
    
    etc.
}