[perl #127974] sprintf() not threadsafe/reentrant if the format tokens use explicit indices

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# New Ticket Created by  Sam S. 
# Please include the string:  [perl #127974]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=127974 >


The following works fine:

    say sprintf '%1$s %2$s', 5, 42;

But the following fails:

    await do for ^100 {
        start {
            say sprintf '%1$s %2$s', 5, 42
        }
    }

    ## Camelia-friendly version:
    # for (^100).race(batch=>1) { sprintf '%1$s %2$s', 5, 42 }

It prints a few successful iterations, but at some point dies with «Your 
printf-style directives specify 0 arguments, but 2 arguments were supplied».

The error only occurs if the format directives specify explicit argument 
indices. If the '%1$s %2$s' format is replaced with '%s %s' in the example 
above, it completes without error.