I can recommend reading perlintro.

Thanks

This doesn't find the second row in the file.

Which file: the one accessed by the file handle  MYFILE or by the  DMPLD handle?

Maybe try writing a program which opens a file, reads and prints each line in a  while loop, and then closes the file. This might be a good foundation for your ultimate goal.

As moritz suggested, you might then want to try splitting each line as you go through the  while loop to access its fields.

[H]ow do I reference the first element of $_ ...

$_ is a scalar; it has no elements. It may be a scalar that is a reference to an array, in which case  $_->[0], $_->[1], ... would access the first, second, etc., elements of the array.

Actually, I just did this to fix it following an earlier suggestion

$id = 1
open(MYFILE,"/var/tmp/mylist.dat") or die "Cant open mylist.dat"

while (<MYFILE>) {
      chomp;
      split/,/;
      die "Request Id $id already present in file\n" if $_[0] eq $id;
                }

close MYFILE;
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$id = 1; # <--- semicolon was missing
open(MYFILE,"/var/tmp/mylist.dat") or die "Cant open mylist.dat"

while (<MYFILE>) { # <--- wrong file handle fixed
      chomp;
      die "Request Id $id already present in file\n" if $_ eq $id;
}

close MYFILE;
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$_, which gets assigned to by <MYFILE> in a while loop, is a scalar.
As such, it can hold a string or a number. The line is not magically split at the comma*. So, $_ just holds the entire line.

The operator eq compares its operands stringwise and returns true if both are equal. Perhaps you want to match?

      die "Request Id $id already present in file\n" if /$id/;
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The match operator m// is by default bound to $_; to match against something else you need the binding operator =~:

while (my $line = <MYFILE>) {
      chomp $line;
      die "Request Id $id already present in file\n" if $line =~ /$id/
+;
}
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If you really want an array and compare $id against its first element, you have to split the line into an array:

    my @array = split /,/, $_;
    die "Request Id $id already present in file\n" if $array[0] eq $id
+;
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Note that split by default splits into the array @_ whose first element is $_[0] - but this usage is deprecated.

*) for magical split, look up the -a, -n, -p and -F switches, and the array @F in perlrun.

I'm a bit puzzled now. I tried your solution and it works fine but so does this

$id = 1; # <--- semicolon was missing
open(MYFILE,"/var/tmp/mylist.dat") or die "Cant open mylist.dat"

while (<MYFILE>) { # <--- wrong file handle fixed
      chomp;
      split /,/;
      die "Request Id $id already present in file\n" if $_[0] eq $id;
}

close MYFILE;
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What is the difference in using an array or doing it as above ? I thought split will split a string into an array of strings anyway. Hence I'm a bit puzzled as to why I explicitly have to use an array as per your example. Is this better programming practice ?

Hence I'm a bit puzzled as to why I explicitly have to use an array as per your example. Is this better programming practice ?

Yes, it is - and precisely because you are puzzled :-)

As I said above, split by default splits into the array @_ whose first element is $_[0] - and it's a deprecated usage because it leads people to be puzzled, and because @_ also is the argument array inside subroutines; so using implicit split inside subs is probably a bad idea.

Oops just realised you answered that by saying it's deprecated. Apologies I should have seen this earlier