Another method... 
use warnings;
use strict;

my $N = 3;
my $new = "NEW";

while (<DATA>) {
    my @words = split /\s+/;
    $words[$N-1] = $new if scalar @words >= $N;
    print join " ", @words, "\n";
}

__DATA__
hi there everyone I am looking to substitute
lots of words for new ones, but only the third
word per line on each line!
one
one two
one two three
one two three four

[download]

output: 

hi there NEW I am looking to substitute 
lots of NEW for new ones, but only the third 
word per NEW on each line! 
one 
one two 
one two NEW 
one two NEW four

[download]

In reply to Re: need help: substitute n-th word in a line by kesterkester
in thread need help: substitute n-th word in a line by star7

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.