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sub upto {
    my $stop = $_[0];
...
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sub frobnicate {
    print "Frobnicating\n";
...
my $thr1 = async { gargle() };
my $thr2 = async { gargle() };
my $thr3 = async { gargle() };

    open my $fh, "<:async", $filename
        or die "Couldn't open '$filename': $!";
    my $line = <$fh>;
    # ... do some busy work without touching $line ...
    print $line;