It's about to get math-y up in here.
This finds which number produces the most terms in the Collatz sequence, up to a value you specify.
For those who don't know, the iterative sequence in the Collatz sequence is:
If n is odd: Multiply by three and add one.
If n is even: Divide by two.
(n*3+1)or(n/2);
Continue until you end up at one.
It's believed every number will eventually end up at one.

#!/usr/bin/perl
use warnings; use strict;

my @longest;
print "Max value: ";
my $length = <STDIN>;
print "[Number::Terms]\n";

for(my $i=2; $i<$length; $i++) {
    my $num = $i;
    my @current;
    while($num > 1) {
        $num = $num % 2 ? (3*$num+1) : ($num/2);
        push(@current, $num);
    }
    if(scalar(@current) > scalar(@longest)) {
        @longest = undef;
        @longest = $current;
        print "$i:" . scalar(@longest) . "\n";
    }
}