perl -le'sub r{join"",map{int rand 10}1..$_[0]}$C=r($l=shift);while($t
+!=$C){print"$t";($r)="$t-$C"=~/^(.*).*-\1/;$t=$r.r($l-length$r)}print
+"$t"' 10
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Some bug-like things:

1. It compares its guesses with the target number using "!=" (shorter than "ne" with spaces), so it goes goofy if you ask for a large number.
2. The first "guess" is an empty string.