Just when I thought nothing could surprise me anymore in Perl, I came across a case in my code that defies the order in which I thought expressions were evaluated. It is demonstrated by the following self-contained program:

use feature qw(say state);

my @widths = (2, 6, 5, 7);
my @partitions = map { state $c = 0; [$c, $c += $_] } @widths;

say '[', join(', ', @$_), ']' for @partitions;

[download]

I would have expected that to print:

[0, 2]
[2, 8]
[8, 13]
[13, 20]

[download]

But it actually prints:

[2, 2]
[8, 8]
[13, 13]
[20, 20]

[download]

... i.e. even though the first $c in the final map statement comes logically (i.e. evaluation-order-wise) before the $c += $_ operation, it is substituted with the value of $c after that operation.

Can someone explain to me why this happens? What am I missing?

In reply to Why does the first $c evaluate to the incremented value in [$c, $c += $_] ? by smls

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