Update: Corion points out that you aren't counting. I think your one-liner might work if you modify your regex to be: 

perl -ne 'print if ($seen{$_} .= @ARGV) =~ /^9876543210$/' FILE1 FILE2

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In reply to Re^2: find common lines in many files by BrowserUk
in thread find common lines in many files by Anonymous Monk

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