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]