That's Perl's syntax, from about 15 years before Groovy, whippersnapper. Off my lawn, now.
Oh, although I guess Perl got that syntax from the Bourne shell…
Huh, I could have sworn that I remembered that Groovy has taken it from Ruby.
Incidentally, Ruby uses just # for variables, it uses #{} for arbitrary expressions. A common style prefers #{} even where # alone works, though.