$x = '1234';
for (substr($x,1,2)) {          # lvalue offset and length start at (1,2)
    $_ = 'a';   print $x,"\n";  # becomes (1,1) -- prints 1a4,
    $_ = 'xyz'; print $x,"\n";  # becomes (1,3) -- prints 1xyz4  
    $x = '56789';               # no change (1,3)
    $_ = 'pq';  print $x,"\n";  # becomes (1,2) after pq is subbed into (1,3) -- prints 5pq9
}