If it works, it works. But all these calculations look rather error-prone to me. I would read the whole header (800 bytes?) in a string and split it by newlines.

my @lines = split /\n/, $header, -1;
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(-1 so that split wouldn't remove trailing newlines, if any)

Then I would find the needed line in @lines and substitute stuff like this:

for my $magic_part ( substr $needed_line, 23 ) {
    die "Line is too short!"
      if length $magic_part < 5;
    $magic_part =~ tr// /c;

    # double magic :)
    substr( $magic_part, 0, 5 ) = "sun4v";
}
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I guess not many people know that substr is magic (and so is foreach loop)... but it works. It's straight from the documentation, so it can't be too bad :) tr works with empty searchlist and "c"omplement option, I don't know why, it's probably better written like this: tr/\x00-\xff/ /

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    ~~
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Then I would join the array with newlines again and overwrite the whole header.

Yes, pretty obscure stuff here, but I'm very bad at math. I try to avoid it as much as I can :)