It's been a while since i've done much Perl, or posted here, so please take a tetanus shot and bear with me, because i'm a little rusty. This might not be the most interesting problem, but i certainly had fun with it...

A Kaprekar number is one where if you square it and break it in half (the right side larger for oddly lengthed numbers), the two halves add up to the original number. This is only roughly the definition, you can go here for a longer layman's definition, and here for a rigorous definition.

One example Kaprekar number: 297
297**2 == 88,209.
88 + 209 == 297

sub kap_check {
  ...
}
[download]

the golf is to fill in the interesting bits that accept one numeric argument and return true or false depending on whether the input is a Kaprekar number or not. If you take a look at the rigorous page, you'll notice some strange numbers that don't fit in with the layman's definition above (like 4879). Ignore those numbers for the purpose of this golf.

#23456789_123456789_123456789_123456789_123456789_123456789_
$_=$_[0]*pop;$==.5*length;/(.{$=})(.*)/;$1+$2==$_**.5
[download]