my %hoa = (
    '01' => [ 50001, 50010, ],
    '02' => [ 50041, 50011, 50301, ],
    '09' => [ 50701, 50801, ],
);

for my $k (keys %hoa){
    my $count = @{ $hoa{$k} };
    print "no of keys in container $k: $count\n";
}