$foo = "x";
  &mod($foo);
  for ($x = 0; $x < 3; $x++) {
      &mod("a");
  }
  sub mod {
    print "before: $_[0]";
    $_[0] = "m";
    print "  after: $_[0]\n";
  }

##</code><code>##

  Modification of a read-only value attempted...

##</code><code>##

  eval { $_[0] .= '' };
  if ($@ =~ /Modification of a read-only value attempted/) {

##</code><code>##

$ perl -MDevel::Peek -le 'Dump("hello")'
SV = PV(0x8121a3c) at 0x812b36c
  REFCNT = 1
  FLAGS = (POK,READONLY,pPOK)
  PV = 0x812f498 "hello"\0
  CUR = 5
  LEN = 6