sub block(&) {
    return undef;
};;

print block {'whatever'} // "is undef";;
String found where operator expected at (eval 10) line 1, near "// "is undef""
        (Missing operator before  "is undef"?)
[Too many arguments for main::block at (eval 10) line 1, near "// "is undef""
syntax error at (eval 10) line 1, near "// "is undef""

print +( block {'whatever'} ) // "is undef";;
is undef

print block( sub{'whatever'} ) // "is undef";;
is undef
"is undef";;