Check CPAN for modules. File::Slurp does this sort of thing, and has just been modified to extend its functions.

use File::Slurp;
my $string = read_file("yourfile");
write_file("yourfile.bak", $string);
$string =~ s/everything/otherthings/g;
$string .= "endbit\n";
write_file("newfile", $string);
[download]

This has problems if your file/string is huge. There is also a Tie::Slurp module or somesuch but I admit I haven't had it working yet.