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

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.