This seems to work if you don't let the engine give any spaces back by switching off backtracking.

$ perl -le '
> print
>    qq{->$_<-: },
>    m{(?x) ^ (?: \z | (?> \s* ) \D ) }
>    ? q{matches}
>    : q{no match}
>    for q{}, q{12ab}, q{cat}, q{  99balloons}, q{   dog};'
-><-: matches
->12ab<-: no match
->cat<-: matches
->  99balloons<-: no match
->   dog<-: matches
$
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I hope this is of interest.