57

#23456789_123456789_123456789_123456789_123456789_1234567
$"='';@s=/./gs;@s=map"@s[($_<$s?0:$_-$s)..$_]",0..$s+$#s;
[download]

59

#23456789_123456789_123456789_123456789_123456789_123456789
$"='';@s=/./gs;@s=map"@s[($_<$s?0:$_-$s)..$_-1]",1..$s+$#s;
[download]

UPDATE
Trying to shorten this, I assumed that the string was in a variable $c, and did this:

@s=map{substr$c,$_<0?(0,$s+$_):($_,$s)}1-$s..length$c;
[download]

This fails. And it fails for reasons that illustrate very clearly why I think that prototypes are officially a Bad Thing. And so my substr based approach comes out at 58 with:

#23456789_123456789_123456789_123456789_123456789_12345678
@s=map{substr$c,$_<0?0:$_,$_<0?$s+$_:$s}1-$s..-1+length$c;
[download]

UPDATE 2
Oops, My initial solution produced an extra character. I am up to 58.