To assign the value to the hash, just do $hosts{$_} = $a;

However, one way you might be able to do this is with an in place edit. Check this thread for inspiration.

--
"I just read perlman:perlboot," said Tom, objectively.
naChoZ

UPDATE: doh! just as I hit submit, the correct term occurred to me, "log resolver"

this one was written for web logs, but I'm sure it would be easily adaptable.