flatten.pl

#!/usr/bin/env perl
use strict;
use warnings;
use v5.10;

my $arr = [[1,2,3],[4,5,[6,7,[8,9,[1,2,3]]]]];
my @arr = ([1,2,3],[4,5,[6,7,[8,9,[1,2,3]]]]);

sub flatten {
  my $arg = @_ > 1 ? [@_] : shift;
  my @tmp = ref $arg eq 'ARRAY' ? @$arg : $arg;
  my @output = map {ref $_ eq 'ARRAY' ? flatten($_) : $_} @tmp;
  return @output;
}

my @output = flatten($arr);
my @output2 = flatten(@arr);

say "@output";
say "@output2";