Joining the content of two files from one-liner (still too long): 
perl -ne 
 'chomp;push@{($|?"_0":"_1")},$_;eof&&$|++;END{$,=$?||map{print$_1[$-+
++],$_,$/}@_0}'
file1 file2

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In reply to join program in one-liner by sh1tn

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