I would like to replace some values in input files named " i\_f.xyz " using perl, where i goes from 1 to 19.

In my files, I want to change a "6" into a "C". 

To do so I tried 

```
perl -pi -e 's/6/C/g' ${i}_f.xyz

```

, and some other combinations but it doesn't work. 

Anyone can help me please ? Thanks a lot :)