my $n = 3;
my $newword = NEW;
my $n_minus_one = $n-1;
while (<DATA>) {
    s/((?:\w+\s+){$n_minus_one})(\w+)/$1$newword/;
    print;
}    

__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
__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]
update - Now that I think about some edge cases like apostrophes and hyphens I would change line to 
s/((?:\w+\s+){$n_minus_one})(\w+)/$1$newword/;

[download]
to 
s/((?:[\w'\-]+[^\w'\-]+){$n_minus_one})(\w+)/$1$newword/;

[download]

--

flounder

In reply to Re: need help: substitute n-th word in a line by flounder99
in thread need help: substitute n-th word in a line by star7

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