my $full_pkg = "Test::".$pkg;
my $hello = $full_pkg->can('hello');
$full_pkg->$hello(...);
# Or: $hello->($full_pkg, ...);

##</code><code>##

my $full_pkg = "Test::".$pkg;
$full_pkg->hello(...);

##</code><code>##

$full_pkg->$method_name(...);