Re: Modify csv file.

[BillKSmith]

It is impossible to modify any type of file "in-place" unless the replacement fields each have exactly the same number of characters as the fields they replace. Even in that case it is usually easier to copy and rename the file as you suggested. The details often be handled automatically. (Refer to runtime options -i and -p in perldoc perlrun)

[clueless newbie]

Actually it's not impossible to modify any type of file "in place". Just for Fun ... to update a file "in place".

...
    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'! $!";
...

Be aware that if something should happen during the update one may be "up the creek without a paddle".