marvell has a correct answer for you. I wanted to explain for you why your choice was not working.

// I am making the assumption that the syntax 
// error here is just a typo ( missing a $ in front
// of the second thes_name) .
$thes_name{$fields[1]} = thes_name{$fields[1]}++;
[download]

What this code does is effectively the following:
Start) $thes_name{$fields[1]} has a value of 1
1) Resolves $thes_name{$fields[1]} to its value "1".
2) Increments $thes_name{$fields[1]} to "2"
3) Sets $thes_name{$fields[1]} to the previously resolved value, "1".
End) Thus $thes_name always stays at "1".

Because you used a "post-increment" it happens after the variable is already resolved. If you had used a "pre-increment", ++$thes_name{$fields[1]}, if would have done what you expected -- but marvells answer for that is better.