If you're going to do this in shell, then you can avoid a bit of work:

$ echo lbn9.pd.cnt | cut -d. -f1,3
lbn9.cnt
[download]

However, since I think you're advocating writing the above in perl (with the backticks), it seems kind of redundant:

join '.', (split '.', $name)[0,2]
[download]

which avoids all the overhead of forking and execing multiple (2-3) subprocesses. Personally, I find the backticks (or even system in general) to be vastly overused in perl, mostly by people pretending it's the shell rather than using it as perl.