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);
}