How do I pipe to Linux split command?

Use - for stdin:

sort -R somefile  | split -l 50 - out

From man split:

Output fixed-size pieces of INPUT to PREFIXaa, PREFIXab, ...; default size is 1000 lines, and default PREFIX is 'x'. With no INPUT, or when INPUT is -, read standard input.

Allowing - to specify input is stdin is a convention many UNIX utilities follow.

out is interpreted as input file. You can should a single dash to indicate reading from STDIN:

sort -R somefile | split - -l 50 out