open FILE, "+<$filename"   or die "I checked a return call: $!\n";
{
    local $/; # Always localize this so it doesn't interfere with other parts of the program.
    $old_data = <FILE>;
}
#do some stuff
seek FILE, 0, 0            or die "Can't seek on $filename: $!\n";
print FILE $new_data       or die "Can't print to $filename: $!\n";
truncate FILE, tell(FILE)  or die "Can't truncate $filename: $!\n";
close FILE                 or die "Can't close $filename: $!\n";