my @requests;
while ( ... ) {
    push @requests, ...;

    if (scalar @requests % 3 == 0) {
        &doRequests(\@requests);
    }
}

sub doRequests {
    my $requests = shift;

    my @coro = map {  
        async {
             # here we get LWP::UserAgent object and to request
        }
    } @$requests;
}