...
    open(my $READER,'+<',$Filename_S)
        or die "Can't open '$Filename_S'! $!";
    open(my $WRITER,'+<',$Filename_S)                      # Don't use
+ '+>'
        or die "Can't open '$Filename_S'! $!";
    # The overflow buffer:
    my @Buffer_a;
    while (<$READER>) {
        # Read (past tense) a line - buffer its replacement ...

        # Modify the line here!
        my $Update_s=...
         
        push(@Buffer_a,$Update_s);
        # Write from the overflow buffer if we can ...
        while (@Buffer_a && length($Buffer_a[0]) < tell($READER)-tell(
+$WRITER)) {
            # Enough room to write $Buffer_a[0] so write it ...
            print $WRITER shift(@Buffer_a);
             };
         };
    # Nothing more to read ...
    close($READER)
        or die "Can't close '$Filename_S'! $!";
    # If there's anything in the buffer write it ...
    while (@Buffer_a) {
        print $WRITER shift(@Buffer_a);
         };
    # Truncate the file, in case, what we're writing is shorter than w
+hat we read
    truncate($WRITER,tell($WRITER));
    #  ... and close
    close($WRITER)
        or die "Can't close '$Filename_S'! $!";
...
[download]

you can do this in two steps:

read open
dump all to a variable (in memory)
apply a substitution to this variable
close
re-open the file, now for writing
print the changed variable to the file (overwrite the whole file)
and close again
[download]

No temp files here, and your file is updated as you want. Use perldoc for the details.

oh well, this seems to be more straightforward solution, thats fine..but can this be done by opening file only once

3. Monks have mentioned the documentation for open
2. Monks provided hyperlinks to the documentation for open
1. Monk specifically mentioned the +< and +> operators

Did you read it?

so its not possible using perl

You specified "no oneliners" and "no temp files". You have not specified what you are going to replace with what nor how big your file is. Under these conditions, I see no general solution (MS Word might use temp files, too, did you know?)