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use strict;
use warnings;
...
B
C
D

while(1){
         last unless my $immed = pop @{$immed_child{$root}};
        get_descends($immed);
        $all_descends{$immed}++;
    }

    foreach my $immed ( @{$immed_child{$root}} )  {
        get_descends($immed);
        $all_descends{$immed}++;
    }