The following works with strict on:

(\&{"Test::${pkg}::hello"})->($pkg);
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The same, but more readable:

my $sub = \&{"Test::${pkg}::hello"};
$sub->($pkg);
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Then again, explicitly using no strict 'refs'; might be better a good thing.

That said, it looks like you have a method call (to which you're passing the wrong package name). If so, you should be using

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