The first argument to split is a regex unless you want paragraph mode (and you don't)

That isn't what is happening here, this is a different special case documented in `perldoc -f split`:

            As another special case, "split" emulates the default beha
+vior of
            the command line tool awk when the PATTERN is either omitt
+ed or a
            string composed of a single space character (such as ' ' o
+r
            "\x20", but not e.g. "/ /"). In this case, any leading whi
+tespace
            in EXPR is removed before splitting occurs, and the PATTER
+N is
            instead treated as if it were "/\s+/"; in particular, this
+ means
            that any contiguous whitespace (not just a single space ch
+aracter)
            is used as a separator. However, this special treatment ca
+n be
            avoided by specifying the pattern "/ /" instead of the str
+ing " ",
            thereby allowing only a single space character to be a sep
+arator.
            In earlier Perls this special case was restricted to the u
+se of a
            plain " " as the pattern argument to split; in Perl 5.18.0
+ and
            later this special case is triggered by any expression whi
+ch
            evaluates to the simple string " ".

            If omitted, PATTERN defaults to a single space, " ", trigg
+ering
            the previously described awk emulation.
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The references to awk are probably not very helpful these days, and probably discourage people from reading the rest and using this useful construct.