My best was 59 characters:

sub p {
map{$_[$_]+=$_[$_+1]for 0..$#_;@_=(1,@_);print"@_\n"}1..pop
}
[download]

However, I can reduce japhy's solution from 56 to 45 characters:

sub p {
@_=(1,map$_[-2]+pop,@_),print"@_\n"for 1..pop 
}
[download]

BTW, I find it amusing that monks are giving their golf subroutines one-letter names, when the subroutine name isn't counted in the length of the solution. :)

I was considering sub pascal's_triangle {} for this one. ;)