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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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