if the 80th character is always the same unique character, e.g. "^"
you can use:
$/ = "^";
and then read in the file as you would a normal line file.

Otherwise read in the file 80 chars at a time with read() and scan the input for non-ascii characters. If you find, for example, 2 non ascii chars, read in two more chars (and check again for more non-ascii chars).

-imran