Scieneer Common Lisp 1.3.9 online documentation

ext:*top-level-auto-declare*[Variable]

This variable controls whether assignments to unknown variables at top-level, or in any other call to eval of setq, will implicitly declare the variable special. These values are meaningful:

:warn
Print a warning, but declare the variable special (the default.)
t
Quietly declare the variable special.
nil
Never declare the variable, giving warnings on each use.