The reason I might use something like Parse::FixedLength (not the module as it stands, though) is that currently I define my fixed width data in one place with something like this (update: and imagine this is 20,50, or 100 fields, which for me makes using just plain unpack unwieldy and unmaintainable without using something like the below):

# fieldname:length
my @format = qw(
 field1:10
 field2:20
 field3:8
);
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Then I have to jump through a couple of hoops (though I admit its not hard) to convert that to an array of just '@fieldnames', and a format string for unpack, and so now my one @format array turns into two arrays and a scalar floating around my code. I think it would save a bit of time and maybe be a bit cleaner to encapsulate that logic in a module (in a way I've suggested earlier), and I just may get around to doing it.

Update: Ok, for good or ill, I've taken princepawn up on his offer to let me take over the Parse::FixedLength module (look for v5.01 in a CPAN near you). Comments/criticisms welcome :-)