perl -we '$string = qq/9500,9501,fred,,9999/; map { warn qq/Bad entry: $_ in string $string/ } grep ! /^\d{4}$/, split /,/, $string;'

##</code><code>##

Bad entry: fred in string 9500,9501,fred,,9999 at -e line 1.
Bad entry:  in string 9500,9501,fred,,9999 at -e line 1.