Seems like you need to construct a second hash which is indexed on the fields you want if you don't want to do this iteratively. You'd need to construct it either by iterating through your current data structure once or at the same time you build this. For example

my %oslookup;
while( my ($pc,$data) = each %hoh ) {
 push @{ $oslookup{$data->{'os'}} }, $pc;
}  

# later on
print "linux pcs are: ", join(',', @{$oslookup{'linux'} || []}, "\n";
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Similarly a second hash for the 'user' field. If you expect to have a lot of keys for each record, with a little creativity you can build a single index hash which has lookups for all the keys in your data structure. When/If this gets really big you might want to change to using a DB structure - either RDBMS or as I prefer DB_File. You'll probably want to use the B-Tree implementation to store multiple values for a key.