If you use Tie::File to 'tie' a file to an array, you can prepend to the file by unshifting to the array.

Now that's a good thought! However, Tie::File can be very slow with large files.

Is there anyway to add the new stuff to the top of the file instead of the bottom?

Perhaps the following will be helpful:

use strict;
use warnings;

addStuffToTopOfFile( "New Stuff!\n", 'File.txt' );

sub addStuffToTopOfFile {
    my ( $stuff, $file ) = @_;

    local @ARGV;
    local $^I = '.bak';
    push @ARGV, $file;

    while (<>) {
        print $stuff if $. == 1;
        print;
    }

    unlink "$file.bak";
}
[download]

File contents before running script:

Line 1
Line 2
Line 3
Line 4
[download]

File contents after running script:

New Stuff!
Line 1
Line 2
Line 3
Line 4
[download]

This solution was implicit in Anonymous Monk's reply, but you send a string and file you want prepended to the subroutine. The script makes, and then deletes, a backup file during the process.

Following up on davido's question somewhat, if what you are trying to achieve is prepending data rather than text, maybe you would be better off considering using a database? They seem scary, or maybe just weird if you haven't used them before, but they can make a lot of data manipulation much easier and faster than trying to figure out how to roll the code from scratch.

What problem are you solving with this?

Would it be adequate to append to the bottom of the file (rather than prepending before the start of the file), and then to read the end of the file with File::ReadBackwards, or File::Tail?

The operation you request is not typically supported by underlying filesystem. OS/400 I think supports record-based filesmembers where you can insert/delete with abandon? For ext4, etc., this operation will be both costly and inherently dangerous.

This said, in case you insist on DWIM, a simple mmap can do the trick, provided it is available.
Update: use map_handle instead of map_file.

no warnings;
use File::Map ':all';

open(my $fh, '+>>', shift) or die; # open/mmap modes must ~~
my $off = $fh->tell;
print $fh join " ", @ARGV;
$fh->flush;

# rotate in place
map_handle(my $mmap, $fh, '+<');
substr($mmap, 0, 0, substr($mmap, $off));
[download]

"Appending" to the beginning of a file. Is that a FAQ?

Indeed it is.

I believe that the relavent FAQ is much easier to find with the tool perldoc.

perldoc -q "append to the beginning of a file"
Bill

In this case, if you have a separate file that contains the sequence in which to read files,
then your problem can be solved by putting the new information first, in the sequence.