In this case what I'd suggest is a custom type constraint.

use Moose::Util::TypeConstraints;

subtype 'TrueStr',
  as 'Str',
  where { !!$_ };

coerce 'TrueStr', from 'Any', via { $_ || "whatever" };

has foo => (is => "ro", isa => "TrueStr", coerce => 1);
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Then any value which is not a TrueStr will be coerced into one via the code $_ || "whatever".

Of course, this also applies to accessors, not just the constructor. (Not a problem for a read-only attribute though!) And also, it means that if you've got a bunch of attributes like this, each with different defaults, you need to define a type constraint for each one.

The alternative - attribute metaprogramming - starts off seeming simple enough, but becomes more complicated when you start thinking about immutability. (Attribute metaprogramming can be a real bitch when it touches on immutability.)

A compromise would be to hook has to automatically build per-attribute type constraints and coecions. I submitted a patch to MooseX::AttributeShortcuts for precisely this feature some months ago, but it was only partly accepted: per-attribute constraints are now supported, but not coercions.