our $O;
sub foo {
	print $O "blah\n";
}
$O = $to_file ? *MYFILE : *STDOUT;
foo();
# OR
sub foo {
	print O "blah\n";
}
*O = $to_file ? *MYFILE{IO} : *STDOUT{IO};
# (the {IO} is not strictly needed but this way we avoid overwriting $O etc)
foo();
# or same as above but like
open O, ">&=", ($to_file ? *MYFILE : *STDOUT);
# or you could even use *MYFILE instead of *O
# OR
sub foo {
	my($P) = @_;
	pri("blah\n");
}
sub pri {
	if ($to_file) { print MYFILE @_ } else { print @_ }
}