my $account;
my @transactions;

while (my $row = <$fh>) {
    my @fields = unpack "a14 a27 .....", $row;

    if ($fields[0] ne "") {
        # New record has account number, so handle the info we've gathered so far.
        process_details($account, [ @transactions ]);

        # Now start collecting information about this new account
        $account = $fields[0];
        @transactions = ();
    }

    # Add transaction details to list
    push @transactions, get_transaction(@fields);
}

# Finally, process the last transaction
process_details($account, [ @transactions ]);

# Handle an account with one or more transactions...
sub process_details {
    my ($account, $aTransactions) = @_;

    # The first time we're called, we won't actually *have* any
    # information, so do nothing
    return unless defined $account;

    # Do what needs to be done...
}

# Fetch transaction details from fields
sub get_transaction {
    my @fields = @_

    # Fetch what you need from the fields and create a transaction

    return $transaction;
}