Why use a hash and sort yourself when the DB can do that already?

select ...
order by account_name, username
[download]

then, in Perl:

my $last_account;
while (my $row = $sth->fetchrow_arrayref({ Slice => {})) {
    if (! defined $last_account or $last_account ne $row->{account_nam
+e}) {
        if (defined $last_account) {
            print "\n";
        };
        $last_account = $row->{account_name};
        print "Account $last_account\n",
    };
    print $row->{username};
};
[download]