while (my ($a,$b,$c) = splice(@data, -3)) {
  push @hashrefs, { a => $a, b => $b, c => $c };
  pop @data;  # null field
}

##</code><code>##

my @hashrefs;
$#hashrefs = int(@data / 4);
my $i = $#hashrefs;
while (@data and my ($a,$b,$c) = splice(@data, -3)) {
  $hashrefs[$i--] = { a => $a, b => $b, c => $c };
  pop @data;
}