$_=@;=1..$a++*$a/2;printf" %@{[y///c]}d"x++$;.$/,splice@;,0,$;while@;
[download]

_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

Thats some nice obfued golfing! I took a few minutes to understand and translate it:

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

my $i;
while(@arr) {
  $i++;
  my $format = ' %' . y///c . 'd'; # y///c is a short version of lengt
+h($_)
                                   # originally in "@{[2 + 2]}" form -
+- which
                                   # creates, dereferences and stringi
+fies 
                                   # 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 u
+sing an
                                   # explicit loop like the rest of us
}
[download]

-Blake

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]

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

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

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

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

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

$#="%@{[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
[download]

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

$#=" %@{[length$a++*$a/2]}g";map{print$j+1..($j+=++$i),$/}1..$a-1
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Based on a few ideas from the other posts I came up with the following improvement of your 65 character version, squeezing it to 62. (Now all I have to go and read the docs and figure out how it works. :-) The one I was working on (and understand) was in the line of hopes latest post which I reduced down to 64.

         1         2         3         4         5         6  
12345678901234567890123456789012345678901234567890123456789012345
#runrig improvement
$#=" %@{[length$a++*$a/2]}g";print$j+1..($j+=++$i),$/for 2..$a
#blakem/hopes/jynx improvement
$;=length$a++*$a/2;map{printf" %$;d",++$,for 2..$_;print$/}2..$a
[download]

Ahhh, Latecomers Advantage. :-)

Yves / DeMerphq
--
Have you registered your Name Space?

$n=1+length$a++*$a/2;map{map{printf"%${n}d",++$k}1..$_;print$/}1..$a-1
[download]

I can trim two chars from that.... to tie japhy at 68

#        1         2         3         4         5         6
#2345678901234567890123456789012345678901234567890123456789012345678
$n=length$a++*$a/2;map{map{printf" %$n.d",++$k}1..$_;print$/}1..$a-1
[download]

-Blake

I can beat 2 characters.
How about this (66 chars):


$n=length$a++*$a/2;map{map{printf" %$n.d",++$k}2..$_;print$/}0..$a
[download]


There are two "\n", I know, but writes exactly what I want... :-)
Hopes

Firstly, you can drop the extra $/'s being printed on yours by changing the 0..$a to 2..$a as shown in #1. The second thing i worked on was a one-liner version, which is #2. It weighs in at 71 characters. However, if i'm allowed to disregard numbers over 1000 (which would take more than 45 rows to get) i can drop that by three characters to 68 as shown in #3.

# 1
$n=length$a++*$a/2;map{map{printf" %$n.d",++$k}2..$_;print$/}2..$a

# 2
print+(map$"x(length(($a+1)*$a/2)+1-length++$l).$l,2..$_),$/for 2..$a+
+1

# 3
print+(map$"x(($a>13?4:$a>3?3:2)-length++$l).$l,2..$_),$/for 2..$a+1
[download]

for(1..$a){printf"%${\length($a*$a/2)}d",$-++;$_==++$?||redo;print$/;$
+?=0}
[download]