as far as i can see, the hash of hashes you create is ok. your issue is how you use the hash later in your code.

%account is a hash, so to get the correct hashref for a given key you would need to do :

my $account_hash = $accounts{78};
then to add a element to the arrayref that the transaction key points to:

push @{$account_hash->{transactions}}, '-42.37';
then to get the data out, use :

my @trans = @{$account_hash->{transactions}}; have not tested my code, so no guarantees against typos.