"SetEnv/SetEnvIf" and mod_perl

Hi again.

Assuming this kind of configuration :

<Location something>
     SetEnvIf Request_URI \.gif$ gif-image
     SetHandler modperl
     PerlResponseHandler MyNiftyModule
..
</Location>

is there any way, in MyNiftyModule, to "get at" this "gif-image" variable ?

Such as, would I find it as $ENV{'gif-image'} ?
What kind of thing are these "Apache environment variables" anyway ?
I have been using them extensively within Apache itself, but I still can't quite figure 
out on what plane they live.  Clearly, they are not "environment variables" in the same 
sense as "cgi-bin environment variables", and not the same thing either as what a 
"PerlsetVar" does, so what are they ?

(And I realise that this is more of an Apache httpd question, but I figure that if someone 
should be aware of the httpd internals, there is a good chance of finding such a person here).