Task 1

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diff --git a/challenge-121/perlboy1967/perl/ch-1.pl b/challenge-121/perlboy1967/perl/ch-1.pl
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+#!/usr/bin/perl
+
+# Perl Weekly Challenge - 121
+# - https://perlweeklychallenge.org/blog/perl-weekly-challenge-121/#TASK1
+#
+# Task 1 - Invert Bit
+#
+# Author: Niels 'PerlBoy' van Dijke
+
+use v5.16;
+use strict;
+use warnings;
+
+use Test::More;
+
+# Prototype(s)
+sub invertBit($$);
+
+my $tests = [ 
+   [ 12, 3,  8],
+   [ 18, 4, 26],
+   [ 0b10101010, 5, 0b10111010 ],
+];
+
+foreach my $t (@$tests) {
+  printf "%s\n", join(',', @$t) unless is(invertBit($t->[0],$t->[1]), $t->[2]);
+  print "------------\n";
+}
+
+done_testing();
+
+sub invertBit($$) {
+  my ($m,$n) = @_;
+
+  # upper part + modified bit + lower part
+  return (((($m >> $n) << 1 ) + ((($m >> ($n - 1)) & 1 ) ^ 1)) << ($n - 1)) + 
+         ($m & (0xff >> (8 - $n + 1))); 
+}