perltrap.pod has an example that shows how perl5 will trap modification of a constant. 
  $foo = "x";
  &mod($foo);
  for ($x = 0; $x < 3; $x++) {
      &mod("a");
  }
  sub mod {
    print "before: $_[0]";
    $_[0] = "m";
    print "  after: $_[0]\n";
  }

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perl5 reports this error as
  Modification of a read-only value attempted...

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

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to try to modify to the constant ahead of time, what other ways does perl provide detect that a parameter to a sub is a constant? I've seen that Devel::Peek can report that the flags of a variable are READONLY. 
$ 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

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Does a module that returns the FLAGS exist?

In reply to Detecting constant arguments passed to subroutines by dcd

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