Retain quotes on CSV fields that were quoted in the input - perl

Question

I have a CSV file such that a few of the fields are quoted regardless of whether they need to be. What I wish to do is load this file, modify a few of the values, and produce the modified CSV with the quoted fields intact.
I'm currently using Perl's Text::CSV package to attempt to solve this problem, but have ran into a bit of a roadblock. The following is a small test script to demonstrate the problem:
use Text::CSV;
my $csv = Text::CSV->new ({'binary' => 1, 'allow_loose_quotes' => 1, 'keep_meta_info' => 1});
my $line = q^hello,"world"^;
print qq^input: $line\n^;
$csv->parse($line);
my #flds = $csv->fields();
$csv->combine(#flds);
print 'output: ', $csv->string(), "\n";
produces:
input: hello,"world"
output: hello,world
According to Text::CSV's documentation, an is_quoted() function exists to test if a field had been quoted in the input, but if I use this to add surrounding quotes to a field, I get unexpected results:
my $csv = Text::CSV->new ({'binary' => 1, 'allow_loose_quotes' => 1, 'keep_meta_info' => 1});
my $line = q^hello,"world"^;
print qq^input: $line\n^;
$csv->parse($line);
my #flds = $csv->fields();
for my $idx (0..$#flds) {
if ($csv->is_quoted($idx)) {
$flds[$idx] = qq^"$flds[$idx]"^;
}
}
$csv->combine(#flds);
print 'output: ', $csv->string(), "\n";
Producing:
input: hello,"world"
output: hello,"""world"""
where I believe the quotes I've added before the combine() are being seen as part of the field, and so are being escaped with a second double quote as combine() is processing.
What would be the best way to ensure quoted fields are left intact from input to output? I'm not certain the application will accept always_quote'ed fields... Is there some combination of Text::CSV object attributes that will allow for keeping quotes intact? Or perhaps am I left with adjusting the record post-combine?

Answer 1

It's a shame but it appears that while keep_meta_info gives you access to the metadata there's no option to tell Text::CSV to reapply the is_quoted state on output.
Depending on how complex your record is you could just reassemble it yourself. But then you'd have to cope with changes to string fields that were previously safely unquoted but after your processing now require quotes. That will depend on the types of changes you introduce, i.e. whether or not you ever expect that a previously "safe" string value will become unsafe. If the answer is "never" (i.e. 0.00000% chance), then you should just do the reassembly yourself and document what you've done.
Post-processing would require that you CSV-parse the string to handle the possibility of commas and other unsafe characters inside strings, so that may not be an option.
Or, you could dive into the code for Text::CSV and implement the desired functionality. I.e. allow the user to force quoting of a specific field on output. I played around with it, and it looks like part of the required mechanism might be in place but unfortunately all I have access to is the XS version, which delegates to native code, so I can't delve deeper at this time. This is as far as I got:
Original combine method. Note the setting of _FFLAGS to undef.
sub combine
{
my $self = shift;
my $str = "";
$self->{_FIELDS} = \#_;
$self->{_FFLAGS} = undef;
$self->{_STATUS} = (#_ > 0) && $self->Combine (\$str, \#_, 0);
$self->{_STRING} = \$str;
$self->{_STATUS};
} # combine
My attempt. I guessed that the second argument to Combine might be the flags, but since the (lowercase) combine API is based on receiving an array and not an arrayref, there's no way to pass two arrays in. I changed it to expect two arrayrefs and tried passing the second to Combine but that failed with "Can't call method "print" on unblessed reference".
sub combine2
{
my $self = shift;
my $str = "";
my $f = shift;
my $g = shift;
$self->{_FIELDS} = $f;
$self->{_FFLAGS} = $g;
$self->{_STATUS} = (#$f > 0) && $self->Combine (\$str, $f, $g);
$self->{_STRING} = \$str;
$self->{_STATUS};
} # combine