I've made a wide exploration of the tree, without recursion, the result is akin the one you've shown on your post

#!/usr/bin/perl
@ini_array   = getarr("1");
@final_array = (1);
while (@ini_array) {
        @aux = ();
        push @final_array, @ini_array;
        map { push @aux, getarr($_) } @ini_array;
        @ini_array = @aux;
}

sub getarr {
        $fl = shift; @sublist=();
        open(A,"$fl.txt") or die "Can't open $fl.txt";
        while (<A>) {
                chomp;
                push @sublist, $_;
        }
        close A;
        return @sublist;
}

print "final: @final_array\n";
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