my %filesystems = (
   hosta => { desc => 'x', filesystemA =>  qq|/the/first/filesystem|, filesystemB => qq|/the/second/fielsystem| },
   hostb => { desc => 'r', filesystemA => qq|/the/first/filesystem|, filesystemB => qq|no filesystemB for 2nd host|}
);


print "filesystemA exists\n" 
  if exists $filesystems{'hosta'}->{'filesystemA'};

print "no_such_fs does not exist\n" 
  if not exists $filesystems{'hosta'}->{'no_such_fs'};