I expect that option 1 would be the most efficient.

Option 2 as originally described doesn't work (but see Tye's response below), because when a match fails, pos gets reset, so it won't be at the end of the string. Instead, you have to set a flag by testing pos against length within the loop:

while (<DATA>) {
  my @accum;
  my $ate_the_whole_thing = 0;
  while (/\G$item_regex/g) {
    push @accum, [$1, $2, $3];
    $ate_the_whole_thing = (pos() == length);
  }
  if ($ate_the_whole_thing) {
    print join ',', @$_, "\n" for @accum;
  }
}
[download]

Option #3 would put a lookahead in the iterative match, which is expensive (as I noted), but does work:

while (<DATA>) {
  my @accum;
  push @accum, [$1, $2, $3] while (/\G$item_regex(?=$item_regex*\Z)/g)
+;
  print join ',', @$_, "\n" for @accum;
}
[download]