>perl -wMstrict -le
"my $funcs = [
   sub { print q{i'm at index zero}  },
   sub { print q{and i'm at index 1} },
   ];
 add_func({
   fns => $funcs,
   fn  => sub { print q{i'm new here} },
   at  => 3
   });
 $funcs->[0]();
 $funcs->[3]();
 sub add_func {
   my %args = %{ shift() };
   $args{fns}->[$args{at}] = $args{fn}
   }
"
i'm at index zero
i'm new here