Two possibilities strike me. First, just replace @foo with ():

  [scalar(()=split(/\t/, $unk{$_})), $_ ]
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That's the =()= operator there, just missing the left equals. Should do what you want the way you're doing it.

Second, any time I see "xxx-delimited", I think "DBD::CSV". I don't know - I just find it more natural to phrase my querying code in a query language. Which, of course, makes it a bit more trivial to move data around without affecting your logic... into a comma-separated file, or into a SQLite database, or mySQL, or Oracle or DB2 or ... maybe it's just me, but I like that flexibility.