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sub c{my$n=-1+shift;$n?map{my$c=$_;map$c.$_,c($n,@_)}@_:@_}

print join " ", c qw(2 a b c d);
print "\n";
...
# outputs
aa ab ac ad ba bb bc bd ca cb cc cd da db dc dd
0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 
+1110 1111