use strict;
use warnings;
use feature 'say', 'state';
use Types::Standard 'Undef';
use Types::Common::String 'SimpleStr','StrLength';
use Type::Params 'compile';

state $check = compile (
    Undef | (SimpleStr & (StrLength[1,4]))->plus_coercions(
        SimpleStr, sub { warn 'coercing'; length ? $_ : undef },
    )
);

#no warnings 'uninitialized';

for my $str ( undef, '', 'abcd', 'abcde' ) {
    my $label = ! defined($str) ? 'undef' : ! length ($str) ? 'empty' : $str;
    my $checked = $check->($str);

    say sprintf('%s : >%s<', $label, $str);
}

##</code><code>##

Use of uninitialized value in sprintf at 11130005.pl line 20.
undef : ><
coercing at 11130005.pl line 10.
empty : ><
abcd : >abcd<
coercing at 11130005.pl line 10.
Value "abcde" did not pass type constraint "Undef|__ANON__" (in $_[0]) at 11130005.pl line 18
    "Undef|__ANON__" requires that the value pass "Undef" or "__ANON__"
    Value "abcde" did not pass type constraint "Undef"
        Value "abcde" did not pass type constraint "Undef"
        "Undef" is defined as: (!defined($_))
    Value "abcde" did not pass type constraint
        "__ANON__" is a subtype of "SimpleStr&StrLength[1,4]"
        "SimpleStr&StrLength[1,4]" requires that the value pass "SimpleStr" and "StrLength[1,4]"
        Value "abcde" did not pass type constraint "StrLength[1,4]"
        "StrLength[1,4]" expects length($_) to be between 1 and 4
        length($_) is 5