I thought I was getting somewhere with this:

...

open (IN, "$data") || die "Couldn't get $data page";
my @all = <IN>;
close (IN);

open (IN2, ">$data.tmp") || die "Couldn't open temporary file";
   foreach my $line (@all) {
    my ($one,$two,$three,$four,$five,$six,$seven) = split "\t",$line;
    if ($three eq $checkthree) {
        ($one,$two,$three,$four,$five,$six,$seven) = @line; last;
        }
   }
   
rename("$data.tmp", "$data") ||  die "Couldn't rename file";

print "okay";
[download]

As it went through and printed "okay".
Unfortunately it wiped the file so obviously my definition of "okay" is different to perl's :-)
What to do?