However, your regex doesn't work

You're right here. I deleted a " by accident. Here's the fix:

$rrstring =~ s/^((?:[^\n";]+|"(?:[^\n"\\]|\\[^\n])*")*);[^\n]*$/$1/mg;
                        ^
                        was missing
[download]

I also disagree with you assertion that the patch won't allow backslashed quotes within TXT RDATA.

It completely ignores backslashes, so I knew there was a problem. I was only wrong in how it fails. Your patch does allow backslashes, but it incorrectly finds the end of the quoted string.

Here's a string where it fails to work:

# Here's the simplest case where it fails:
# $expect = "\"Joe\"";
# $rdata  = $expect . " ; \"comment\"";

$expect = "\"John \\\"The Wiz\\\" Doe; programmer\"";
$rdata  = $expect . " ; \"comment\"";

print("Input:  ", $rdata,  $/);
print("Expect: ", $expect, $/);

if(($rdata)=~m[;]) {
                if(($rdata)=~m[^(".*")($|\s*;)]) {
                    $rdata = $1;
                } else {
                    ($rdata)=~s/;.*//g;
                }
            }

print("Get:    ", $rdata, $/);

__END__
output
======
Input:  "John \"The Wiz\" Doe; programmer" ; "comment"
Expect: "John \"The Wiz\" Doe; programmer"
Get:    "John \"The Wiz\" Doe; programmer" ; "comment"
[download]

btw, the /g in ($rdata)=~s/;.*//g; is not necessary.