if (grep($seen{$_}, @SupportedPTs) )
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translates into English as: if any (non-zero returned by grep) of @SupportedPTs are keys in %seen (with corresponding true values).

You want "if any of @SupportedPTs are NOT keys in %seen":

if (grep(! $seen{$_}, @SupportedPTs) )
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(with your if and else blocks reversed) or perhaps "if none (zero returned by grep) of @SupportedPTS are NOT keys in %seen":

if (! grep(! $seen{$_}, @SupportedPTs) )
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with the if and else blocks unchanged.