@A_K = grep /^[A-K]/, @query_results;
@L_Z = grep /^[L-Z]/, @query_results;

##</code><code>##

my @A_K;
my @L_Z;
my $aref = \@A_K;
while ( my $row = $dbh->fetchrow_arrayref ) {
    $aref = \@L_Z if ( $$row[$last_name_col] =~ /^L/ );
    push @$aref, [ @$row ];
}