How do I randomly update a binary file?

If you're just trying to patch a binary, in many cases, something as simple as this works: 

    perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs

However, if you have fixed sized records, then you might do something more like this: 

    $RECSIZE = 220; # size of record, in bytes
    $recno   = 37;  # which record to update
    open(FH, "+<somewhere") || die "can't update somewhere: $!";
    seek(FH, $recno * $RECSIZE, 0);
    read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
    # munge the record
    seek(FH, $recno * $RECSIZE, 0);
    print FH $record;
    close FH;

Locking and error checking are left as an exercise for the reader. Don't forget them, or you'll be quite sorry.

Don't forget to set binmode under DOS-like platforms when operating on files that have anything other than straight text in them. See the docs on open and on binmode for more details.