I have a file with two numbers 

```
4     4
4      4
9         8
7     7

```

Each line may have a different number of spaces.
What I would like to do is use `perldo` to compute the difference of the two numbers:

I am trying right now

```
:perldo @a=split(/ +/, $_); $_ =  $a[0] - $a[1]

```

I think this should work but the file is not changed at all.

How can I do this using perldo?