$a=5;
print map{@_=$j+1..($j+=++$i);"@_\n"}1..$a

#Update 2 chars better (39)
print map{"@{[$j+1..($j+=++$i)]}\n"}1..$a

##</code><code>##

$#="%@{[2*length $a]}d";print map"@{[$j+1..($j+=++$i)]}\n",1..$a
$#="%@{[2*length $a]}u";print map"@{[$j+1..($j+=++$i)]}\n",1..$a
$#="%@{[2*length $a]}s";print map"@{[$j+1..($j+=++$i)]}\n",1..$a

##</code><code>##

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

##</code><code>##

$#=" %@{[length int$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

##</code><code>##

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