sub prepare_foo_print {
    my ($foo, $file) = @_;
    open my $fh, '>>', $file or die "bummer: $!\n";
    my $oldfh = select $fh;
    # currently opened $file now is default filehandle
    # (instead of STDOUT)
    print_whatever ($foo);
    select $oldfh;
}
sub print_whatever {
    print shift;
}