use strict;
use warnings FATAL => 'all';
use Test::More;

sub distance
{
    length $_[0];
}

sub answer
{
    $_ = $_[0];
    return $1 + distance($_) if s/\(#:(\d+)\)/#/;
}

is(
    answer('Life, the universe & everything? (#:3) Huh?'),
    42,
);

done_testing;
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Update: you've posted the whole code now. I get the same error. Another solution (other than 0+$1) is to quote the variable, "$1".

Update #3: Now you've added some simplified code and I can see what's going on! That code actually runs more or less how I'd expect it to run.

The problem is that you're expecting that on the first iteration, $1 will be evaluated before the recursive call to butter however that is not necessarily the case. With the exception of the short-circuiting operators (&&, || and so on), Perl does not guarantee what order the two operands of a binary operator will be evaluated in.

For example:

use strict;
use warnings FATAL => 'all';

sub foo {
    print "foo just happened\n";
    40;
}

sub bar {
    print "bar just happened\n";
    2;
}

print foo + bar, "\n";
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What happens? You might expect that you'd get output like this:

foo just happened
bar just happened
42
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The "correct" behaviour here is undefined. As it happens, on my machine the first two lines are reversed.

So your problem is that the recursive call to butter is evaluated before $1. The second call of butter ends up setting $1 to 6, overwriting the existing value 7. Blammo! 6+6 is 12.

Quotes around $1 or using a subexpression like (0+$1) coerces Perl into changing the evaluation order, and (by luck more than anything else) you get the correct answer.

Update 4: the C FAQ (C is a programming language with similar undefined behaviour here) has this related entry.

Update 5: this is very fragile stuff. If you add a prototype of () to the foo function in my example, then it changes the output order (at least it does here). If take away the prototype but call foo with parentheses, like foo() then that has the same effect.

Interesting SoPW post though. Kept me entertained. Much thanks.