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 return is already present
    print $oh $_;
}
close $ih;
close $oh;
rename ( $tmp, $file );