Because we have no clue how this hash is being constructed, nor do you show anything you've tried, I'll post here a (imho) better method of constructing your hash, and how to get what you want. Your HoA is very inefficient for the type of lookups you want to do, so I'd translate it into something more along the lines of:

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";
}
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-stevieb