hm.p6

multi sub trait_mod:<is>(Sub $s, :@in){
    @in.append($s);
    say @in; # roughly [something]
}

my @funcs = [];

sub something is in(@funcs) { 1; }

say @funcs; # []

zz.py

# desired effect in python

funcs = []

def add_to(listy):
  def _add_to(func):
    listy.append(func)
    return func
  return _add_to

@add_to(funcs)
def funcy():
  return 1

print(funcs) # [funcy]