I get:

syntax error at noname.pl line 2, near "%hash{"
Execution of noname.pl aborted due to compilation errors.
[download]

with:

my %hash = (hello => "Hello World\n", my_function => \&my_func);
my $func_ref = %hash{"my_function"};

$func_ref-> (\%hash);

sub my_func {
    my $hashref = shift;
    
    print $hashref->{hello}
}
[download]

so it seems the sample code you provided is not quite the same as the code you tested, or there is a Perl version difference issue perhaps? (I'm using AS Perl v5.8.7)