You have warnings on! Why don't you read the warning you get? Useless use of exponentiation (**) in void context means that your $j**3 line isn't doing anything with its return value. Perhaps you meant $j **= 3 which would set $j to itself cubed.

I assume you have a sum() function defined somewhere. And your variable scoping leaves something to be desired -- specifically, proper variable scoping. Rewriting only the scoping of your program, I'd do: 

my @numbers;

for my $i (100 .. 999) {
  my @num = split(//, $i);
  foreach my $j (@num) {
    $j **= 3;
  }
  if (sum(@num) == $i) {
    push @numbers, $i;
  }
}

[download]
I have another comment. Why did you use 'for' in one loop and 'foreach' in another? They're not different things, and you're not using the C-style loop, so there's no reason one of them should have a different name from the other.

All in all, I would rewrite this program to use far fewer temporary variables: 

my @numbers = grep { sum(map { $_ ** 3 } split //) == $_ } 100 .. 999;

[download]
That might be a bit too succinct for you, but it does the same thing as yours.
Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart

In reply to Re: Cube/digit script. by japhy
in thread Cube/digit script. by Andrew_Levenson

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.