... better written as
   die "Unknown char! \n" unless grep { exists $testHash{$_} } @test;

This will only  die when none of the characters in the test string are 'known':

>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.
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A better way to write this (assuming something like Array::Diff is not acceptable) might be:
    exists $testHash{$_} or die "Unknown char! \n" for @test;