my $line = "x" x 23 . "something";

for my $magic ( substr $line, 23 ) {
    print length $magic, "\n";
    $magic =~ tr/\x00-\xff/ /;
    substr( $magic, 0, 5 ) = "sun4v";
}

print "~~$line~~", "\n"
__END__
9
~~xxxxxxxxxxxxxxxxxxxxxxxsun4v    ~~