How to use the flag -p of BSD split?
The issue is caused by BSD split being later in your path than the GNU split (which doesn't support the -p option). If you do an echo $PATH you should see this - /usr/local/opt/coreutils/libexec/gnubin will be before /usr/bin in the output.

To call BSD split directly, use the full path - pipe to /usr/bin/split instead of split.

Your original command will then become:


seq -w 1 1 10 | gsed ':a;N;$!ba;s/\n//g' | split -p '060'