$string = "abcdefghijklmnopqrstuvwxyz0123456789";

$txt = substr($string,0,10);
say($txt);                   
               # abcdefghij

$txt = substr($string,10,10);
say($txt);
               # klmnopqrst

$txt = substr($string,-10,10);
say($txt);     
               # 0123456789

$txt = "Mein Name ist Bond";
substr($txt,14,4,"James");
say($txt);
               # Mein Name ist James

$txt = "Mein Name ist Bond";
substr($txt,14,0,"James ");
say($txt);
               # Mein Name ist James Bond

$txt = "Mein Name ist Bond";
$txt .= ", James " . substr($txt,14,4);
say($txt);
               # Mein Name ist Bond, James Bond

$txt = "1234567890";
substr($txt,5) = "*****";
say($txt);     # 12345*****

$txt = "1234567890";
substr($txt,5,0) = "\n";
say($txt);     # 12345
               # 67890



sub say 
  { print @_, "\n"; }