I just taken your input as array and done the below program

 use strict;
use warnings;
use Data::Dumper;

my @test=qw( [ a b ] c d [ e [ f g h [ i j ] k l ] m n ] o p );

my ($append, $opencount, @temp, $closecount);

foreach(@test){

    if($_ eq '['){
        ++$opencount;
        $append.=$_;
    next;
    }

    if($_ ne ']' && $opencount){
        $append.=$_;
        print "----$append----\n";
        next;
    }

    if($_ eq ']'){
        --$opencount;
        $append.=$_;

        unless($opencount){
            #print "------------=>$append<=---------------";
            push @temp, $append;
            $append=undef;$closecount=0;
            $opencount=0;
            next;
        }
        next;
    }

    else{
        #print "------------=>$_<=----------------";
        push @temp, $_;
    }

    #print Dumper \@temp;
}
print Dumper \@temp;
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