I don't see how the size of the file prevents you from opening it. Perhaps you meant something else. If you show us the code you're using, we can probably point you in the right direction.
As far as "masking" goes, substr would seem to be an appropriate tool:
$ perl -E 'my $x = "1234567890123456"; say $x; substr $x, 6, 6, "*"x6; + say $x' 1234567890123456 123456******3456