You could use grep instead of a loop to filter the characters.

use strict;
use warnings;

my %allowed  = map { $_ => 1 } q{a} .. q{o};
my @testStrs = qw{ acbbedfydaccdtg abcde };
my @unknown  = ();

foreach my $testStr ( @testStrs )
{
    @unknown = grep { ! $allowed{ $_ } } split m{}, $testStr;
    print
       qq{String - $testStr: },
       @unknown
       ? qq{Unknown characters found: @unknown\n}
       : qq{Success\n};
}
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The output.

String - acbbedfydaccdtg: Unknown characters found: y t
String - abcde: Success
[download]

I hope this is of use.