unless (($key =~ /^,|,,|,$/))

Cute, testing for the negative is much more readable here ... but probably something I wouldn't have tried. You need to check for the characters being in the valid class though (and I'd also put brackets around the anchors) so that would make it...

 unless (($key =~ /(?:^,)|,,|[^a-zA-Z,]|(?:,$)))
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Which isn't quite as nice anymore :(. So I'd probably still use the "obvious" non-negative...

 if (($key =~ /^[a-zA-Z]+(?:,[a-zA-Z]+)*$))
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Or if I was feeling really special, I might even do...

 my $field = qr([a-zA-Z]+);
 if (($key =~ /^$field(?:,$field)*$))
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...which is slightly more readable IMO.

update (broquaint): changed <pre> to <code> tags