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sub foo {
    my $o = "";
...
} else {
    print MYFILE foo();
}

sub foo {
    print "blah\n";
...
}
foo();
$OLDH and select $OLDH;

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

sub foo {
    my($P) = @_;
...
}
foo($to_file ? sub { print MYFILE @_ } : sub { print @_ });
# or foo(sub { if ($to_file) { print MYFILE @_ } else { print @_ } });

our $O;
sub foo {
...
sub pri {
    if ($to_file) { print MYFILE @_ } else { print @_ }
}