use strict;
use warnings;

$_ = '[1, [2, 3, 4, [5, 6]]]';
my @a;

sub e{my($r)=@_;{s/^\]//&&last;s/^\[//&&e($r->[@$r]=[])||s/^, //||s/^(\d+)//&&push@$r,$1and redo}}e\@a;

use Data::Dumper;
print (Dumper(\@a));