The PubMed.pm file needs to be located in WWW\Search e.g

c:\perl\site\lib\WWW\Search\PubMed.pm
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This is because perl searches for modules like so - splits on the :: of the given package name, where everything up to the last element is a directory and the last element is the name of the file with a .pm automatically appeneded and then this path is search for in the one of the @INC directories (left-to-right) until found. In code it might look like this

use File::Spec::Functions;

sub use {
  my($pkg, @args) = @_;

  my(@dirs, $file) = split '::', $pkg;
  my $path = catfile @dirs, "$file.pm";

  eval q<
    BEGIN {
      my($lib) = grep { -f cafile($_, $path) } @INC;
      my $code = IO::File->new(catfile $lib,$path) or die "ack - $!";
      $INC{$path} = catfile $lib, $path;
      eval $code->getlines;
      $file->import(@args);
    }
  >;
}
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You can find further details in perlmod.
HTH

_________
broquaint