57 chars:
  $t=$_;@s=(map({substr$t,0,$_}1..$s-1),/(?=(.{1,$s}))/gs);

57 chars, string in $t:
  @s=map{substr$t,$_>0&&$_,$_<0?$s+$_:$s}1-$s..-1+length$t;

55 chars, string in $t:
  @s=(map({substr$t,0,$_}1..$s-1),$t=~/(?=(.{1,$s}))/gs);

55 chars:
  @s=/./gs;@s=map{join'',@s[$_>0&&$_..$_+$s]}- --$s..$#s;