In your specific example you have at least two options:

1. Rummage through package stashes and globs e.g

   @Foo::ISA = qw(Hello World);
   
   use strict;
   use feature 'say';
   
   my $class = 'Foo';
   my $isa   = *{ $main::{"$class\::"}{ISA} }{ARRAY};
   
   say "@$isa"; # Hello World
   [download]

2. Use the core package Symbol and make use of qualify_to_ref e.g

   @Foo::ISA = qw(Hello World);
   
   use strict;
   use feature 'say';
   use Symbol 'qualify_to_ref';
   
   my $class = 'Foo';
   my $isa   = *{qualify_to_ref 'ISA', $class}{ARRAY};
   
   say "@$isa"; # Hello World
   [download]

The former won't work with nested packages whereas the latter will.