Hmm I am not sure I understand either the question or the answers! The way I read it was you wanted to comment out all lines which have the word test in them, which would look like this (also putting the close outside the loop and creating a new file rather than appending which also seems wrong!: 
#!/usr/bin/perl -w
my $file_name = "test.txt";
open(INF,'./$file_name') || die "file could not open\n";
open(INFN,'>./${file_name}.new') || die "$!: ${file_name}.new\n";
my @line;
while(@line = <INF>) {
        if ( $line =~ /test/) {
            print "found at $.: $line
            $line = "\# $line";
        }
        print INFN $line;
}
close(INF);
close INFN;

[download]
If you really want to append to the file, read it in, close it and then reopen it for append. Anyway, it is usually wrong to open or close the same file inside a loop.

One world, one people

In reply to Re: find & replace a string in perl by anonymized user 468275
in thread find & replace a string in perl by vr786

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.