Also, I'm very curious how those files are dealt with by various programs. I guess it seemed like something that should be easy to do, but then became a challenge

AFAIK such a file will be generated by creating an ID3 v2.4 tag first, and then trying to modify it with WMP11 (or possibly WMP12, which I haven't tested yet). WMP11's inbuilt tag editor doesn't support v2.4, and what is worse, it even can't recognize the existence of a v2.4 tag. It eventually produces a "double-tagged" file as a result.

But obviously it's just a bug of WMP which should not happen. Once you've layered a v2.3 tag over v2.4, you're no more able to access the original v2.4 tag.

Oh and let me say hello. I'm very new to this forum

#!/usr/bin/perl -wt
use strict;
use Fcntl;

sysopen(MP3, "test.mp3", O_RDONLY|O_BINARY) || die "Can not open file\n";
my $headers = 0;
while (read(MP3, my $buffer, 100)) {
  $buffer = unpack("H100", $buffer);
  my $index = index($buffer, "494433");
  if ($index > -1 && $index < 74) {
    if (hex(substr($buffer, $index + 7, 2)) < 0xff &&
        hex(substr($buffer, $index + 9, 2)) < 0xff &&
        hex(substr($buffer, $index + 13, 2)) < 0x80 &&
        hex(substr($buffer, $index + 15, 2)) < 0x80 &&
        hex(substr($buffer, $index + 17, 2)) < 0x80) {
        $headers += 1;
    }
    if ($headers > 1) {
      die "Chained ID3v2 headers found.\n";
    }
  }
}

close(MP3);

#!/usr/bin/perl -wt
use strict;
use Fcntl;

sysopen(MP3, "test.mp3", O_RDONLY|O_BINARY) || die "Can not open file\n";

my $headers = 0;
while (read(MP3, my $buffer, 100)) {
  $buffer = unpack("H100", $buffer);
  my $index = index($buffer, "494433");
  if ($index > -1 && $index < 74) {
    my $header = substr($buffer, $index, 20);
    if (hex(substr($header, 7, 2)) < 0xff &&
        hex(substr($header, 9, 2)) < 0xff &&
        hex(substr($header, 12, 2)) < 0x80 &&
        hex(substr($header, 14, 2)) < 0x80 &&
        hex(substr($header, 16, 2)) < 0x80 &&
        hex(substr($header, 18, 2)) < 0x80) {
        $headers += 1;
    }
    if ($headers > 1) {
      die "Chained ID3v2 headers found.\n";
    }
  }
}

close(MP3);