Your problem is that when you print $_, you're printing the entire line. You want to just print the part of the regex that matched. The easiest way to do it is to capture that part of the regex with parentheses, then print out the captured portion: 
#!/usr/bin/perl -w

use strict;

while (<DATA>)
{
  if (/\s(159\.230\.\d+\.\d+)/)
  {
    print $1,"\n";
  }
}

__DATA__
network-object 159.230.123.000 255.255.224.0
network-object 159.230.234.000 255.255.252.0
network-object 159.230.111.000 255.255.255.0
network-object host 159.230.101.000
network-object host 159.230.200.000
network-object host 159.230.6.000

[download]

In reply to Re: Match only certain IP Addresses by sgifford
in thread Match only certain IP Addresses by monger

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.