Heres my initial entry at 56: It take the 'at least one space between numbers' condition to its logical conclusion.

sub g {
#        1         2         3         4         5
#2345678901234567890123456789012345678901234567890123456
$^=pop;for(1..$^){printf"%$^d",++$,for(0..$@++);print$/}
}

=g(3) OUTPUT
  1
  2  3
  4  5  6

=g(5) OUTPUT
    1
    2    3
    4    5    6
    7    8    9   10
   11   12   13   14   15

=g(7) OUTPUT
      1
      2      3
      4      5      6
      7      8      9     10
     11     12     13     14     15
     16     17     18     19     20     21
     22     23     24     25     26     27     28
[download]

Update: with the param already in $a, I can cut it down to an even 50....

$a = 5;
#        1         2         3         4         5
#2345678901234567890123456789012345678901234567890
for(1..$a){printf"%$a.d",++$;for(0..$@++);print$/}
[download]

Update2: Jumping through the new rules clarification (and borrowing the cleverly ordered calculation) I'm at 70....

#        1         2         3         4         5         6         7
#234567890123456789012345678901234567890123456789012345678901234567890
$b=length$a++*$a/2;for(2..$a){printf" %$b.d",++$;for(0..$@++);print$/}
[download]

Final update before I have to go

63 chars... follows the letter (if not the spirit) of the new rules. ;-)

#        1         2         3         4         5         6
#23456789012345678901234567890123456789012345678901234567890123
map{map{$a=0if$_>$a-2;printf" %$a.d",++$;}0..$@++;print$/}1..$a
[download]

-Blake