The problem is ## HELLO

$ perl -MO=Deparse,-p grimy
sub test_a {
    use warnings;
    use strict 'refs';
    (my $arg = shift());
    (ref($arg) ? $$arg->test_a([0]) : $arg); ## HELLO
}
sub test_b {
    use warnings;
    use strict 'refs';
    (my $arg = shift());
    (ref($arg) ? test_a($$arg[0]) : $arg);
}
use warnings;
use strict 'refs';
test_b([42]);
test_a([42]);
grimy syntax OK
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The problem is indirect object syntax, to fix you need a forward declaration ( sub test_a; ) or parenthesis