>perl -wMstrict -le
"my %testHash = map { $_ => 1 } 'a' .. 'j';
 for my $str (@ARGV) {
   my @test = split '', $str;
   die qq{unknown char in '$str'}
     unless grep { exists $testHash{$_} } @test;
   print qq{'$str' ok};
   }
"  abcd  axyz  xyz
'abcd' ok
'axyz' ok
unknown char in 'xyz' at -e line 1.