Some slight tweaking is in order. The backslashes are not needed, since &foo is already a reference in scalar context. You don't need the dot to dereference the routine value either, though it doesn't hurt anything. You don't need the last $ if you bind to &plus_op_subref instead. Also, one thing you couldn't know, since I just changed it, is that all operator names now use a variant of hash subscript syntax, so the name of the builtin addition operator is any of:

    infix:«+»
    infix:<<+>>
    infix:{'+'}
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So I'd currently write your code as:

    my &plus_op_subref = &infix:«+»;
    my &not_op_subref  = &prefix:«!»;

    say plus_op_subref(3, 2); # 5
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See http://www.wall.org/~larry/syn for preprints of the most recent Synopses. And http://www.wall.org/~larry/apo has Apocalypses annotated with "Update" sections.