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for ('00'..'89'){
  if (index($str,$_)<0) {
    if (index($str.substr($_,0,1),$_)<0) {
...
  }
}
print $str.'90';

$s = (<>);
for('00'..'99'){print $_." missing\n" if $s!~/$_/}
for('00'..'99'){print $_." duplicated\n" if index($s,$_) >= 0 and inde
+x($s,$_,index($s,$_)+1) >= 0}