First off you should check out perldsc, the Perl Data Structures Cookbook. It contains discussion and good examples of how to make many perl datastructures (lists of lists, list of hashes, hashes of lists, hashes of hashes, etc...).

Here's some quick code that defines the hash, inserts an element, and extracts an element out of the structure. Hopefully this will be enough to get you started along the path to Perl datastructure enlightenment:

my %users;
$users{foo}=[{a=>1,b=>2},{a=>2,b=>3}];
my $x=$users{foo}[1]{a};
#$x==2 at this point
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