To change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file

The general solution is to create a temporary copy of the text file with the changes you want, then copy that over the original.

    $old = $file;

    $new = "$file.tmp.$$";

    $bak = "$file.bak";

    open(OLD, "< $old")         or die "can't open $old: $!";

    open(NEW, "> $new")         or die "can't open $new: $!";

    # Correct typos, preserving case

    while (<OLD>) {

        s/\b(p)earl\b/${1}erl/i;

        (print NEW $_)          or die "can't write to $new: $!";

    }

    close(OLD)                  or die "can't close $old: $!";

    close(NEW)                  or die "can't close $new: $!";

    rename($old, $bak)          or die "can't rename $old to $bak: $!";

    rename($new, $old)          or die "can't rename $new to $old: $!";

Perl can do this sort of thing for you automatically with the -i command-line switch or the closely-related $^I variable . Note that -i may require a suffix on some non-Unix systems; see the platform-specific documentation that came with your port.

    # Renumber a series of tests from the command line

    perl -pi -e 's/(^\s+test\s+)\d+/ $1 . ++$count /e' t/op/taint.t

    # form a script

    local($^I, @ARGV) = ('.bak', glob("*.c"));

    while (<>) {

        if ($. == 1) {

            print "This line should appear at the top of each file\n";

        }

        s/\b(p)earl\b/${1}erl/i;        # Correct typos, preserving case

        print;

        close ARGV if eof;              # Reset $.

    }

If you need to seek to an arbitrary line of a file that changes infrequently, you could build up an index of byte positions of where the line ends are in the file. If the file is large, an index of every tenth or hundredth line end would allow you to seek and read fairly efficiently. If the file is sorted, try the look.pl library (part of the standard perl distribution).

In the unique case of deleting lines at the end of a file, you can use tell() and truncate(). The following code snippet deletes the last line of a file without making a copy or reading the whole file into memory:

        open (FH, "+< $file");

        while ( <FH> ) { $addr = tell(FH) unless eof(FH) }

        truncate(FH, $addr);

Error checking is left as an exercise for the reader.