Perhaps you could either form the hash key out of the hostname and filesystem, something like 'host1:/foo', 'host1:/bar', 'host2:/baz' etc, or you could key by hostname and have each value be a hashref keyed by filesystem.

I don't fully understand what you're doing, however, based on your code sample... There are a number of problems I can see at first glance:

• "hosta => 1" is a string; did you want it to be a hashref? Same with "hostb => 2".
• Avoid barewords like \/file\/system\/a; put in single quotes instead, like '/file/system/a'.
• You try to put a hashref in $filesystems{1}{filesystem}, but it has an odd number of elements. Hashes and hashrefs should contain only key => value pairs.
• You need a $ sigil on host in the last line.