There are methods for writing to a file you are reading, but it is much simpler to write to a temporary file and move it over the original when done, e.g.: 
my $file = 'fred.txt';
my $tmp = 'fred.txt.tmp' . $$; # habitually I make tmp files unique in
+ case of multiuser usage.

open my $ih, $file;
open my $oh, ">$tmp";

while ( <$ih> ) {
    /My_Name/ and $_ = '## ' . $_ . "\n" ; # although a carriage retur
+n is already present
    print $oh $_;
}
close $ih;
close $oh;
rename ( $tmp, $file );

[download]

One world, one people

In reply to Re: Help in read and write to file at a same time in same file. by anonymized user 468275
in thread Help in read and write to file at a same time in same file. by gskoli

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.