# assuming INFILE and OUTFILE are already open,
# and string variables are already set...

my $match =
    $old_username . "\x00"x(84-length($old_username)) .
    $old_password . "\x00"x(84-length($old_password));

my $replace =
    $new_username . "\x00"x(84-length($new_username)) .
    $new_password . "\x00"x(84-length($new_password));

$/ = undef;
my $filedata = <INFILE>;
$filedata =~ s/$match/$replace/;

print OUTFILE $filedata;