Ruby might have taken that from Perl, which also implicitly returns the last evaluated statement in a subroutine.
It seems like the longer I am in software development, I find out that more and more things came from Lisp.
Not wrong that ruby took a lot from perl, but it was heavily influenced by lisp and smalltalk and I'd personally argue those are the two largest influences.
It does seem that ruby was specifically designed to grab Perl programmers though. Several features match exactly.