That's obvious.

Defining an anonymous sub won't execute it but return the code-ref.

  DB<39> p sub { print "run @_" }      # returns ref
CODE(0x2e80a90)
  DB<40> sub{ print "run @_" }->(1..3) # calls ref
run 1 2 3
  DB<41>
[download]

Though I'm a bit surprised, my muscle memory wanted brackets around the ref for precedence:

( sub{ print "run @_" } )->(@_)

this could still be needed in other contexts. (or - more likely - it's just cargo cult from JS ;)