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use feature qw(say state);

...
my @partitions = map { state $c = 0; [$c, $c += $_] } @widths;

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

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

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