sub foo {
	my $o = "";
	$o .= "blah\n";
	$o;
}
if ($to_file == 0) {
	print foo();
} else {
	print MYFILE foo();
}

##</code><code>##

sub foo {
	print "blah\n";
}
my $OLDH;
if ($to_file) {
	$OLDH = select MYFILE;
}
foo();
$OLDH and select $OLDH;

##</code><code>##

sub foo {
	my($O) = @_;
	print $O "blah\n";
}
foo($to_file ? *MYFILE : *STDOUT);

##</code><code>##

sub foo {
	my($P) = @_;
	&$P("blah\n");
}
foo($to_file ? sub { print MYFILE @_ } : sub { print @_ });
# or foo(sub { if ($to_file) { print MYFILE @_ } else { print @_ } });

##</code><code>##

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 @_ }
}