Or worse, the $1 variable will have the value of the previous time it matched and it will overwrite the existing entry in the hash.

Hm. For some reason, I was under the impression that $1 and friends would be undef after any unsuccessful match. Oh, wow, it doesn't; gross:

$ perl -lwe '$_="foo";
             /(\w+)/ && print "match: $1";
             /(\W+)/ || print "no match: $1";'
match: foo
no match: foo
[download]

In this case it wil overwrite it with the same value it already had as all values of the hash are set to "blah" (which is utterly strange).

Well, I just assumed that the original poster was using that "blah" as a placeholder for something else, for the purposes of posting. The presence of quite a few other syntactic errors makes it clear that this was not the code that actually ran...