For reasons which might not be intuitively obvious, the broken behavior is required

I previously regarded /usr/bin/env as 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 -w which works almost everywhere but not in FreeBSD :(