A simple command is a sequence of optional variable assignments followed by blank-separated words and redirections, and terminated by a control operator. The first word specifies the command to be executed, and is passed as argument zero. The remaining words are passed as arguments to the invoked command.
So it's perfectly legal to write:
foo=bar echo $foo
but it doesn't work as I expect (it prints just a newline). It's quite strange to me since:
$ foo=bar printenv foo=bar TERM=rxvt-unicode [...]
Could someone please explain me where I'm doing wrong?