I would just go ahead and make this an explicit loop like below rather than messing with the do{} construction. Also your example just has letters. If that's what you really have instead of multi-character tokens, there is an easier way to do this with tr as shown below.

#!/usr/bin/perl -w
use strict;
$|=1; #autoflush so die comes out in right printout order
      #(after the prints)

my @allowed   = split '', 'abcdefghijklmno';
my %testHash; 
@testHash{@allowed}=();
my @test = split '', 'acbbedfydaccd';

#die "Unknown char! \n" unless exists $testHash{$_} for (@test);

foreach (@test)
{
   print "Unknown char! $_ \n" unless exists($testHash{$_});
}
####### another way #########
###### use tr to count characters not in the set of allowed

my $test = 'acbbedfydaccdy';
my $errors = $test =~ tr/abcdefghijklmno//c;
print "\nerrors = $errors\n";
print "test=$test\n";

die "died Unknown character\n" if ( $test=~tr/abcdefghijklmno//c );

__END__
Unknown char! y 

errors = 2
test=acbbedfydaccdy  # I added another y at end for testing
died Unknown character
[download]