Personally, I'd go with:

sub shall_we {
    my $ans = prompt(
        "What'll it be?",
        -menu => { 'Yes, definitely.' => 1,
                   'No way.'          => 0 },
    );
    return $ans ? 0+$ans : undef;
}
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not that I disagree with your expectation after a quick initial read of the docs

Or even after a thorough read. The returned object is not documented at all. From peeking at the source,

• It returns whether prompt was successful (whatever that means) when used in boolean context.
• It returns the answer as a string when used in string context.
• It returns the answer as a number when used in numerical context.

It has no methods.

It has a very weird destructor that clobbers $_ if you don't do one of the above before the object is destroyed. That allows the following to work:

sub shall_we {
    local $_;
    prompt(
        "What'll it be?",
        -menu => { 'Yes, definitely.' => 1,
                   'No way.'          => 0 },
    );
    return $_;
}
[download]

Update: Fixed ... : ... : ... that should have been ... ? ... : ...