If you want to modify the array in place without creating a new array you can use splice. Because that is altering the number of elements in the array, you have to work in from the right.

use strict;
use warnings;

my @arr = qw{[ A ] B C [ D ] E F [ G ] H I};

my $idx = $#arr;
while ( $idx >= 2 )
{
    if ( $arr[$idx] eq q{]} )
    {
        splice
           @arr,
           $idx - 2,
           0,
           join q{}, splice @arr, $idx - 2, 3;
        $idx -= 3;
    }
    else
    {
        $idx --;
    }
}

print qq{$_\n} for @arr;
[download]

The output is

[A]
B
C
[D]
E
F
[G]
H
I
[download]

I hope this is useful.

Cheers,

JohnGG