It seems that the problem is that you're implicitly loading PDL::index here instead the perl function index. Don't load the PDL::index unless you think that you will need it

#!/usr/bin/perl -w
use strict; 
use PDL::Core;
use PDL::Index;
use PDL::Basic;
use PDL::Math;
use PDL::Primitive;
use PDL::Slices qw/!index/; # <-HERE is the problem (and the solution)
use PDL::IO::FITS;

  my $test = "This is a test and I hope it works.";
  my $pos4 = index $test,"test";
  print "pos4 = $pos4\n";
  exit(0);   

__END__

pos4 = 10
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Updated: In fact it was a little more complicated, really PDL::Index was not the culprit at the end, they are three different index here:

the perl function index
a PDL function index in the module PDL::Slices (the real culprit)
and a module named PDL::Index
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