$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;


##</code><code>##

      die "Request Id $id already present in file\n" if /$id/;

##</code><code>##

while (my $line = <MYFILE>) {
      chomp $line;
      die "Request Id $id already present in file\n" if $line =~ /$id/;
}

##</code><code>##

    my @array = split /,/, $_;
    die "Request Id $id already present in file\n" if $array[0] eq $id;