$_   = ($a*($a+1))/2;   # calculate last number in triangle
my @arr = 1..$_;        # list of all numbers in triangle

my $i;
while(@arr) {
  $i++;
  my $format = ' %' . y///c . 'd'; # y///c is a short version of length($_)
                                   # originally in "@{[2 + 2]}" form -- which
                                   # creates, dereferences and stringifies 
                                   # a one element array ref

  my @row = splice(@arr,0,$i);     # take $i elements off the front of @arr
  printf $format x $i . $/, @row;  # print the whole row, instead of using an
                                   # explicit loop like the rest of us
}