You can't get there from here

Anything

This argument works just as well to show that
- We can't recognize functions that add two numbers together

          my $newcode = q{sub { my $f = eval "CODE";
                                $f->("INPUT");
                                return $_[0] + $_[1];
                               }};

We can't recognize functions that solve the knapsack problem

          my $newcode = q{sub { my $f = eval "CODE";
                                $f->("INPUT");
                                return solve_knapsack(@_);
                               }};

We can't recognize functions that will commit invalid array accesses

          my $newcode = q{sub { my $f = eval "CODE";
                                $f->("INPUT");
                                @a = (1..3); $z = $a[142857];
                               }};

Etc.
- Because if we could, we could also recognize functions that halt
- And we can't recognize functions that halt