/usr/bin/envas being portable onto any sane unix platform.
But now I've had to work with FreeBSD and my beliefs are dashed.
FreeBSD /usr/bin/env implements what posix says, rather than what everyone else does. That leads to trouble when you want to pass a parameter in a shebang, like this:
#!/usr/bin/env perl -wwhich works almost everywhere but not in FreeBSD :(