So your problem is that the recursive call to butter is evaluated before $1.

Quotes around $1 or using a subexpression like (0+$1) coerces Perl into changing the evaluation order,

Both of those statements are completely false. In all existing builds of Perl, the LHS of addition is always evaluated before it's RHS, so $1 is evaluated first in $1+f(). Keep in mind that $1 evaluates to a scalar, not a string. It's "+" that extracts the string from it, by which point the scalar's value has changed.

Study the following:

$_ = 3;
print sub :lvalue { print "$_+";       $_ }->() +
      sub :lvalue { print "10="; ++$_; 10 }->(), "\n";
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3+10=14
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You can clearly see $_ being evaluated before 10 even though the value to which $_ is set by the RHS is used by the addition.

(Sorry for making a chain of post. I initially assumed your post was correct and that it just had a small problem, so I commented on it before reading on. And then the same thing happened another time. Only after that did I realize the entire post is wrong.)