perl.h: White-space only
Revert the removal of 'use encoding'

We've decided to not remove 'use encoding' in v5.20.

This reverts commits b017fe877ad78f04afaa3c1327fc46391f572bfb,
7053d92917f7cb46452de86dc4c6d8644cae849c, and
b06e47f717f77453fbdaef628efc2b9c83898a8b