Week 221 Task 2

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diff --git a/challenge-221/cheok-yin-fung/perl/ch-2.pl b/challenge-221/cheok-yin-fung/perl/ch-2.pl
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+# The Weekly Challenge 221
+# Task 2 Arithmetic Subsequence
+# Brute-force approach, just hope the number of lines is minimalist
+use v5.30.0;
+use warnings;
+use List::Util qw/all sum uniqint/;
+use List::MoreUtils qw/slide/;
+
+sub as {
+    my @arr = @_;
+    return 1 if scalar @arr == 1;
+    my $len_of_longest_as = 2;
+    for my $i (reverse 7..2**($#arr+1)-1) {
+        my $l = length sprintf("%0b", $i);
+        my @bin = split "", "0" x ($#arr+1-$l).sprintf("%0b", $i);
+        next if sum (@bin) < $len_of_longest_as;
+        my @n_arr = map {$arr[$_]} grep {$bin[$_]} (0..$#arr);
+        my @diffs = slide {$a-$b} @n_arr;
+        $len_of_longest_as = sum (@bin) if scalar (uniqint @diffs) == 1;
+    }
+    return $len_of_longest_as;
+}
+
+use Test::More tests=> 3;
+ok as(9, 4, 7, 2, 10) == 3;
+ok as(3, 6, 9, 12) == 4;
+ok as(20, 1, 15, 3, 10, 5, 8) == 4;