Here's what I decided on: 
   if(defined($o)) {
    $o=substr(pack("N",$o | ($t{$string}<<12)),1,3);
    print F2 $o; undef $o;
   } else {
    $o=$t{$string};
   }

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I thought about "L" and chop, but thought N might be more portable. And for unpack: 
 $r=read(F1, $double, 3); ($double)=unpack(N,"\000$double");
 $old_code=$double & 0xfff; $double >>= 12;

[download]

In reply to final decision: the whole cookie! by jhanna
in thread 12 bit ints pairs into 3 bytes by jhanna

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