# Skip the builtins, they are all kept and it doesn't hurt to have
# them but they are already handled.
my @fds = grep { $_ > 2 } IO::Async::OS->potentially_open_fds();
# Keep them all open. Parens are to make the parser trust that
# map was given a code reference and not a hash reference
my @setup = map { ("fd$_" => 'keep', ) } @fds;

# ... later ...
my $thing = IO::Async::Whatever(
    code => sub { ... },
    setup => \@setup
);