Sure you can, BUT the new record (line) must be exactly the same length as the old one. For example, if you just want to comment out a line by placing a '#' at the front then you cannot overwrite the existing line since that would be one extra character - you would overwrite the first character of the next line in the file.

Here is a simple example where the new line is the same length as the old:
use warnings;
use strict;
use Fcntl qw(SEEK_CUR);

# open existing file for read/write
open (my $in, '+<', 'file1.txt') || die "Unable to open file1.txt:$!";

while (<$in>) {

    # Identify the record
    if (/4444/) {
         # Construct the new record
         s/4/#/g;

         # Position the file pointer
         seek ($in, -length($_)-1, SEEK_CUR);
         print $in $_;
    }
}

close $in;

[download]
Input file was:
111111111111111
22222222
33333333333
4444444
5555555555555555
66666666666

[download]
file after processing is:
111111111111111
22222222
33333333333
#######
555555555555555
66666666666

[download]

In reply to Re^5: substitute on file line by line not working syntax error by cdarke
in thread Search and Replace line by line not working on Filehandle by matze77

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