$#=" %@{[length int$a*$a/2]}.f";print map{$j+1..($j+=++$i),$/}1..$a
# or
$#=" %@{[length$a++*$a/2]}.f";print map{$j+1..($j+=++$i),$/}1..$a-1
# or
$#=" %@{[length$a*($a+1)/2]}.f";print map{$j+1..($j+=++$i),$/}1..$a