Why not let Perl do the buffering for you. read and write are buffered (unlike sysread and syswrite). 
while (my $bufsize = read(INFILE, my $eight_byte_chunk, 8)) {
   ...[ Add padding    ]...
   ...[ Encrypt        ]...
   ...[ Merge with IV  ]...
   ...[ Compute new IV ]...
   write(OUTFILE, $eight_byte_chunk, 8);
}

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In reply to Re: Fast parsing for cypher block chaining by ikegami
in thread Fast parsing for cypher block chaining by fluffyvoidwarrior

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