You're checking if string value defined. And it's certainly defined. Try the following:

use strict;
use warnings;

use Test::More tests => 7;                      
my $val = 'Mysimple_mod';

BEGIN { use_ok('Mysimple_mod', qw (external) ) };
ok ( defined( &external ) ,           "#2 external is defined");
ok ( defined( &Mysimple_mod::internal ) , "#3 internal is defined");
ok ( defined( &Mysimple_mod::bogus ) ,        "#4 bogus subroutine");
ok ( defined( &{$val.'::bogus'} ) ,        "#5 bogus subroutine");
ok ( defined( &{$val.'::internal'} ) ,        "#6 internal subroutine"
+);

my $catval = "${val}" .'::bogus'; 
print "\n$catval\n";
ok ( defined( &$catval ) ,                     "#7 bogus subroutine");
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