Here's the code:

  use Devel::Symdump;

  my $x = Mod::idx('some string');
  print "x=$x\n";  # works, x=5

  my $obj = Devel::Symdump->new('Mod'); # crash on 5.10

  package Mod;

  use constant 'STRING' => 'string';

  sub idx {
    my $s = shift;
    return index($s, STRING);
  }
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The crash happens in Devel::Symdump (version 2.08, line 55), where this code is executed:

  local(*ENTRY) = $val;
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If I print $val it reads: GLOB(0x1fa64e4)

Further investigation lead me to constant.pm (1.17, line 106):

  if ($symtab && !exists $symtab->{$name}) {
      # No typeglob yet, so we can use a reference as space-
      # efficient proxy for a constant subroutine
      # The check in Perl_ck_rvconst knows that inlinable
      # constants from cv_const_sv are read only. So we have to:
      Internals::SvREADONLY($scalar, 1);
      $symtab->{$name} = \$scalar;
      mro::method_changed_in($pkg);
  } else {
      *$full_name = sub () { $scalar };
  }
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As far as I can tell this is an optimization in 5.10.

If I change the if-condition (0 && $symtab ...) to force the "else" branch, the crash does not happen.

$val then reads: *Mod::STRING

Lucky me, I found a solution for my module. Of course I didn't want to remove pod-coverage from the test suite (and I didn't want to switch to Readonly or plain variables or to use a regex instead of index). If I force the constant into scalar context, it works, too:

  index($s, scalar STRING)
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$val then reads (with unmodified constant.pm): SCALAR(0x1fa65ec)

I replaced the constant in index() with a simple function showing the state of wantarray(). I think the context is always scalar, so using scalar() should make no difference. But it does in the situation above!?

At this point I came to the edge of my knowledge and decided to head for the wisdom of the monks (I've been registered here for more than a year but never posted something until now. But I've read many, many marvellous posts. Thanks, monks!)