Of course aesthetics always depends on personal taste, but I like the following solution, which has a "functional" flavour:

print(
 (length(join('',map { $mystring =~ /$_/ ? '' : 'X' } @a)) 
 ?   'not ' 
 :   ''),
 "all letters occur\n");
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The idea is to build a string which consists of as many X as there are missing characters. Of course, if you don't like using =~ - maybe because @a might contain characters which have a special meaning in regexp context - you could also write the block inside the map as:

index($mystring,$_) < 0 ? 'X' : ''
[download]