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.