Of course open BAYES, "tail -f bayes.out |" or die "could not open: $!"; would work even better as it lacks the subtle precedence problem that makes the error checking get optimized away . . .

(Or one could be fashionable and go for a 3-argument, lexical filehandle form open my $bayes, "-|", qw( tail -f bayes.out ) or die "Couldn't open pipe: $!"; if you're into that kind of thing and have a recent enough perl.)