Here's a variation that doesn't use ++, and shows the same behaviour: 
use 5.010;
my $x;

sub X {
    $x = shift;
   \$x;
}

say "${X(1)}${X(2)}";
say "${X(1)} ${X(2)}";
__END__
22
1 2

[download]

In reply to Re^3: Order of evaluation/interpolation of references by JavaFan
in thread Order of evaluation/interpolation of references by Anonymous Monk

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