It is a valid way, and is Perl's way of doing 'Multidimensional hashes' without hitting hashes of hashes. Admittedly $; isn't normally space, but such is.
In Programming Perl 2nd Ed this is all on Page 133, my 3rd Ed. is at home at the moment I'm afraid so I can't give page reference in that.
The following code actually works.. which is somewhat scary when it's simply not What You Mean, as I don't think it is in this case.
#!/usr/bin/perl -w
use strict;
$ENV{'Paul', 'Golds'} = 'Molt';
print $ENV{'Paul', 'Golds'};
