What's discouraged (much of the time)
Only by some people
is often more costly than using the Perl equivalent/variant.
So what? In many cases, it doesn't matter.
Note also he's using sed to strip off some of the output of ps. I'm sure someone made a module somewhere that mimics all the flavours of ps, but I rather spend a few microseconds forking and execing a 'ps', then I go hunting down said module, download and install it, and learn how it works.
There can be all kinds of reasons one does something in Perl instead of shelling out. But being considered "discouraged" by some doesn't strike me as an important reason.
But that may be just me.