gistfile1.txt

my @data;
my $sth = $dbh->prepare( $sql ) or die $dbh->errstr;
$sth->execute;
while ( my @row = $sth->fetchrow_array ) {
    push @data, [@row];
}

open my $fh, '>', $flat_file or die $!;
print $fh @{$_},"\n" for @data;
close $fh;

# now better:

open my $fh, '>', $flat_file or die $!;
my $sth = $dbh->prepare( $sql ) or die $dbh->errstr;
$sth->execute;
while ( my @row = $sth->fetchrow_array ) {
    print $fh @row,"\n";
}
close $fh;