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";
}