How do I 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?

Although humans have an easy time thinking of a text file as being a sequence of lines that operates much like a stack of playing cards--or punch cards--computers usually see the text file as a sequence of bytes. n general, there's no direct way for Perl to seek to a particular line of a file, insert text into a file, or remove text from a file.

(There are exceptions in special circumstances. Replacing a sequence of bytes with another sequence of the same length, for example. Another is using the $DB_RECNO array bindings as documented in the DB_File manpage. Yet another is manipulating files with all lines the same length.)

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 (see -i and $^I for more details). 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).