sub collatz_seq {
    my $input = shift;
    my $n = $input;
    my $result = 0;
    my $new_n;

    while ($n != 1) {
        $result += $cache[$n], last
            if defined $cache[$n];

        $n % 2 ? ( $result += 2, $new_n = (3 * $n + 1) >> 1 )
               : ( $result += 1, $new_n = $n >> 1 );

      # $cache[$n] = $cache[$new_n] + ($n % 2 ? 2 : 1)
      #     if defined $cache[$new_n] and $n < $max;

        $n = $new_n;
    }

    $cache[$input] = $result if $input < $max;
    return $result;
}

##</code><code>##

# Intel i7 laptop, Docker Container, Ubuntu + Perlbrew Perl 5.30.1

collatz3_a.pl 1e7  32.291s  (a) original, accepts argument
collatz3_b.pl 1e7  30.134s  (b) a + replaced division with >> 1
collatz3_c.pl 1e7  28.503s  (c) b + removed 1 level of branching
collatz3_d.pl 1e7  21.464s  (d) c + reduced loop iterations
collatz3_e.pl 1e7  19.357s  (e) d + caching less

# AMD 3970x, Docker Container, Ubuntu + Perlbrew Perl 5.30.1

collatz3_a.pl 1e7  13.130s  (a) original, accepts argument
collatz3_b.pl 1e7  12.394s  (b) a + replaced division with >> 1
collatz3_c.pl 1e7  12.261s  (c) b + removed 1 level of branching
collatz3_d.pl 1e7   9.170s  (d) c + reduced loop iterations
collatz3_e.pl 1e7   7.681s  (e) d + caching less