I personally believe I have some problems understanding your question: I'm very tired so it may be just me, but I'm trying to recap from your description of the problem, and please tell me if I'm getting anything wrong...

1. "@validbufs is a unique list of strings." (Taken verbatim!) Thus not regexen...
2. You need to "print out the lines in BUFFER.dat that have the first column (in that line) exactly matching any element in the array @validbufs." (Also verbatim but for a typo.) Thus what does it mean to "exactly match" in this context?

I think I eventually got it: you want to print those lines in your input file i.e. BUFFER.dat such that their first field is an element of @validbufs. Is this a correct rephrasing of your problem? If so, then the general rule is to always make that @validbufs into a hash, say %isvalid. (Although in 5.10 times you can take advantage of the smartmatch operator ~~, but I'm sure that for a problem of this size a hash is still better.) This can be just as simple as:

my %isvalid;  # outside of the loop over lines, of course!
@isvalid{@validbufs}=(1) x @validbufs;
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Then you'd simply

print $line if $isvalid{$field};
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with $field got as per ikegami's suggestion or perhaps not even created as an intermediate variable.

Now, it is to be said that since as you claim there is a 1-1 correspondence between lines that match in the file and entries of @validbufs, one may be tempted to delete from %isvalid any key that matches, to shrink it, because once it has matched it won't match any more. But given the amount of work this would inflict on %isvalid and hashes' efficiency at lookups, I'm sure that both a big-Oh analysis and experimental evidence would be that it would take more work overall than not doing it...