There is no answer in the general case of how that naked block could be avoided, but I find that do can often help group things like this better. (In this example, you could use a ternary so the do isn't even necessary, but I'll humour you and assume it stands for a more complex piece of code.)

{
    my $bar = do {
        if( $foo ) { "true" } else { undef }
    };
    print $bar;
}
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