use v5.16;
use warnings;

# collapse a list 

my @data = (0, 3, 8, 9, 5, 2, 0, 1, 0);

my $acc = 0;

say join(',',@data);

while (@data && $data[0] < 5) {
  $acc += shift @data;
}
$data[0] += $acc;

$acc = 0;
while (@data && $data[$#data] < 5) {
  $acc += pop @data;
}
$data[$#data] += $acc;


say join(',',@data);