You're missing some semi-colons:

my $subref = $tests;  # Right here

$testsub = sub {
        print $_;
        };  # And here
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You're assigning the anon sub to a variable, so the statement is being parsed as an expression that happens to contain a subroutine, not as a subroutine on its own.

----
I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated