Update: Damn it! Never mind; the penny dropped. It's the same order of evaluation thing there's been a couple of recent threads about: $d[ $pB-1 ] = $d[ $pB++ ]

There's no guarantee that $pB-1 will be evaluated before $pB++; even though the post increment guarantees that the preincrement value will be used for the second subscript.

Why is this:

my $pB;
for( $pB = $topB+1; $pB < $topC && $d[ $pB ] < $temp; $d[ $pB-1 ] = $d
+[ $pB++ ] ) { };
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Not equivalent to this?

my $pB;
for( $pB = $topB+1; $pB < $topC && $d[ $pB ] < $temp; $d[ $pB-1 ] = $d
+[ $pB ], $pB++ ) { };
[download]