As you asked for generic comment in your OP, a few remarks

# modules are usually not "use"d inside subs, but just once at the cod
+e start
use Text::CSV;

sub _moredata_array
{
    my ($datastring, $datasep_cfg) = @_;

    # Do not/never quote variables if you do not explicitly want to fo
+rce PV context
    # e.g. if $datasep_cfg is undefined is means something completely 
+different then
    # the empty string which it is forced to when quoted
    my $csv = Text::CSV->new ({ binary => 1, auto_diag => 1, sep_char 
+=> $datasep_cfg });

    # you can declare $io as lexical (my) in the open call itself
    # At this point,Text::CSV->error_diag () is undefined, as $csv has
+ not even been used
    # and opening of the scalar as file handle does not interact with 
+$csv (yet)
    open my $io, "<:encoding(utf8)", \$datastring or die "Cannot use C
+SV: $!";

    # no need to declare a variable
    # the return value can possible be a reference to an empty list. t
+hink if you want
    # that. || undef is useless here
    return $csv->getline ($io);
    }

sub _moredata_hash
{
    my ($datastring, $hashsep) = @_;
    my $csv = Text::CSV->new ({ binary => 1, auto_diag => 1, sep_char 
+=> $hashsep });

    open my $io, "<:encoding(utf8)", \$datastring or die "Cannot use C
+SV: $!";
    my %hash;
    while (my $row = $csv->getline($io)) {
        my $count = scalar @{$row};
        $count % 2 and die "$count is an odd number of keys and values
+ at (@{$row}).\n";
        $hash{$row->[0]} = $row->[1];
        }
    scalar %hash ? \%hash : undef;
    }
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