Just playing, may not work in your character set:

perl -le "while(1){$a=$a?$a:ord'a';print chr$a;$a=++$a%ord'{';sleep 1}
+"
[download]

This probably will though

perl -le "$_='x';while(1){s/aa/a/;print$_++;sleep 1}"
[download]

Cheers,
R.