A challenge was made in comp.unix.shells for a short program to detect common lines in both fileA and fileB, printing one copy of each of common lines. Of course, this was a natural one-liner in Perl, but I stumbled across a bizarre way which I though you might appreciate.
perl -ne 'print if ($seen{$_} .= @ARGV) =~ /10$/' fileA fileB >output

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In reply to Detect common lines between two files, one liner from shell by merlyn

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