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);
  }