I was hoping you'd reply Toby, thank you.

I made a couple of changes to the code so it compiles, and added a test to see that one can pass in the empty string and it will pass validation by being coerced to undef. But the syntax to do this must be wrong:

ok( $Caption->check( $Caption->coerce('') ), 'Empty string passes' );
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... I feel sure that there's a way to avoid calling coerce manually.

Here's the revised test script:

use Types::Standard qw( Maybe Str );
use Types::Common::String qw( StrLength );

my $NonEmptyCaption = StrLength[ 1, 10 ];

my $EmptyStr = Str->where( q{ $_ eq '' } );

my $Caption = (Maybe[$NonEmptyCaption])->plus_coercions(
    $EmptyStr => 'undef',
);


use Test::More;

ok( $Caption->check('My pic'),               'Good caption passes' );
ok( ! $Caption->check('This is too long'),   'Bad caption fails' );
ok( $Caption->check(undef),                  'Undef passes' );
ok( ! $Caption->check([]),                   'Arrayref fails' );
is( $Caption->coerce(''), undef,             'Empty string coerced to 
+undef' );
ok( $Caption->check( $Caption->coerce('') ), 'Empty string passes' );


done_testing;
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Outputs:

perl 11130005-3.pl

ok 1 - Good caption passes
ok 2 - Bad caption fails
ok 3 - Undef passes
ok 4 - Arrayref fails
ok 5 - Empty string coerced to undef
ok 6 - Empty string passes
1..6
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Wonder if there is a more idiomatic way to do the check when a coercion is used. Thanks!