IO::Prompt is returning an instance of IO::Prompt::ReturnVal, not a plain scalar value. Apparently this instance isn't boolean-ifying the way you expect (not that I disagree with your expectation after a quick initial read of the docs) and since it's not false you always get your first leg of your if executing.

Update: As a workaround you can explicitly stringify return $ans . "" or numify it return $ans + 0 and it should behave the way you want. Alternately explicitly test the value (e.g. if( shall_we() == 1 ) { ... } else { ... }).

Another way: Since you're using 5.10 there's always given:

use feature qw( switch say );

## ... as your code ...

given( shall_we() ) {
    when (1) {
        print "In the 'if' statement: yes, we shall.\n";
    }
    when (0) {
        print "In the 'if' statement: no, we shant.\n";
    }
};
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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 $_;
}
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Update: Fixed ... : ... : ... that should have been ... ? ... : ...

However, it looks like this -raw flag never made it to the current version of IO::Prompt.