Make sure that your web server can read and write the file that you are using for passwords.

Also my personal approach to this kind of problems is to read the complete file first, then modify it, and then to write it back. Something along these lines:

open (IN,"<$myfile") or die "open failed for read: $!";
my @lines = <IN>;
close (IN);

open (OUT, ">$myfile") or die "open failed for write: $!";
foreach (@lines) {
  chomp;
  my ($user,$pass) = split(/=/);
  # Check username
  if ($user eq $i_user) {
    # Check password
    if ($pass eq $i_pass) {
      # Check the confirmation of the new password
      if ($i_newpass eq $i_cnewpass) {
        print OUT "$user=$i_newpass\n";
      }
    }
  }
  else {
    print OUT "$user=$pass\n";
  }
}
close (IN);
[download]

These, of course, looks childish, but it's extremely easy to read and follow. If this one works, then you can rewrite it in a more compact way. :)

P.S.: Code is untested of course. :)

Leonid Mamtchenkov aka TVSET