You could use Types::Standard, where it's fairly easy...

use Types::Standard qw( ArrayRef ConsumerOf );

..., isa => ArrayRef[ ConsumerOf["SomeRole"] ], ...
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Or you can use Moose::Meta::TypeConstraint and Moose::Util::TypeConstraints something like (untested):

isa => find_type_constraint("ArrayRef")->parameterize(role_type "MyRol
+e")
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