You might also be interested to investigate the setbuf and setvbuf methods of IO::Handle.

Problem with those is that they're usually not available starting with Perl 5.8.0, because they rely on stdio, while newer perls are typically built with perlio...

$ perl -MIO::Handle -e'IO::Handle->new->setvbuf($buf,1,1024)'
IO::Handle::setvbuf not implemented on this architecture at -e line 1.
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(But it's still recommended to read the respective C man page for setvbuf - even if you can't use that functionality from Perl :)  - as it gives a nice introduction to buffering in general, i.e. block-buffered vs. line-buffered vs. unbuffered, etc.)