Assuming that $pos and $poss are two different things, and $pos is defined somewhere outside of the code snippet, you can save a bundle by doing something like this:

my $docpos = $pos{$docID};
for (1 .. $tf) {
  $docpos->{vec ($dp, $index++, 32)}=\$order;
}
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It may seem trivial, but that's about 2000000*avg_tf dereferences.
Getting rid of an unnecessary "my" variable should also pick up a few extra cycles.