Use coercion.

use Types::Standard qw(Any Enum);

has error_mode => (
  is     => 'rw',
  isa    => (Enum[qw/carp error both/])->plus_coercions(Any, q{'both'}
+),  # default to 'both'
  coerce => 1,
);
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(Coercions can also be done with native Mouse types, but that requires more lines of code, and I can't remember how off the top of my head.)