Here is my solution of 44 chars.

#23456789_123456789_123456789_123456789_1234
map{$y=$_;map{$s[$_].=$y}$c..$s+$c++}/(.)/g;
[download]

It still has the problem of repeating the entire phrase if the window ($s) is larger than the length of the string. In order to correct that, I came up with a 65 character solution of

#23456789_123456789_123456789_123456789_123456789_123456789_12345
@t=/(.)/g;$s=@t<$s?@t:$s;map{$y=$_;map{$f[$_].=$y}$c..$s+$c++}@t;
[download]

Dave