I notice that bar will return 1 if you introduce a scalar context explicitly.

sub bar{
    if (1){
        return scalar do {
            #do block will return 1
            if (1){
                1;
            }
            1;
        };
    }
}
[download]

I notice also that B::Deparse turns 'if(1)' into do, so you also get '1' if you give the optimizer more to chew on:

sub bar{
    my $truth = 'beauty';
    if ($truth){
        return do {
            #do block will return 1
            if ($truth) {
                1;
            }
            1;
        };
    }
}
[download]

My guess is that the scalar context is getting lost in all those nested do blocks, but even so, I can't see why that would be a problem. It looks like a bug to me.