The problem with this kind of stuff (such as nc host port | tr foo bar) is usually that the first program (the one you read from) will buffer output if the stdout isn't a terminal, and you can do nothing about that without changing that program. Just changing the second program isn't enough.
I don't know tcpdump so I can't tell if that's the problem here too though.
There might be a command line option to make tcpdump output non-buffered. Looking at the man page, -U (packet-buffered) looks promising. Alternatively, take a look at -c (exit after receiving count packets) and -G (rotates the dump file specified with the -w option every rotate_seconds second).
I would also like to point out that there's a good Perl module for Pcap, which is what tcpdump is built around: Net::Pcap.
And it's no exception here. There's no guarantee that enough other packets will be gathered to fill the buffer in that second. Or maybe you're trying to make sure the child finishes, but close already does that by calling a blocking waitpid.