use Data::Dumper;
$hash{a1}{b1} = 'c1';
$hash{a1}{b2} = 'c2';
$hash{a2}{b1} = 'c3';
$hash{a2}{b3} = 'c4';

print Dumper(\%hash);


foreach $key (keys %hash){
    print "Key:\t$key\n";
    my %subHash = %{$hash{$key}};
    print "\tSubkeys:";
    foreach $subkey (keys %subHash){
	print "\t$subkey";
    }
    print "\n";
}