I think I would use a coercion to simplify that, and, yes, shed the unwanted attributes (accessors). Note I dropped the signatures as I don;t have the Filter module installed, but also no longer needed. 

package Moo::Role::DBIConnection {
    use Moo::Role;
    use DBI;

    has 'dbh' => (
        is       => 'ro',
        required => 1,
        coerce   => sub {
            my $args = shift;
            return $args if ref($args) eq 'DBI::db';
            ref($args) eq 'HASH' or die 'Not a DB handle nor a hashref
+';
            return DBI->connect( @{$args}{qw/dsn user password options
+/} );
        },
    );
};

[download]
##
# testing

package MyClass {
    use Moo;
    with 'Moo::Role::DBIConnection';
};

use Test::Most 'die';

my %args = (
    dsn      => 'dbi:mysql:database=mysql',
    user     => 'ntonkin',
    password => undef,
    options  => { RaiseError => 1 },
);

subtest 'With no args' => sub {
    dies_ok sub { my $o = MyClass->new }, 'exception on no args';
};

subtest 'With bad type' => sub {
    throws_ok { my $o = MyClass->new(dbh => [\%args]) } qr/Not a DB ha
+ndle nor a hashref/;
};

subtest 'With existing handle' => sub {
    my $dbh = DBI->connect( @args{qw/dsn user password options/} );
    cmp_ok( $dbh->do('select count(*) from db'), '>', 0, 'Found a DB' 
+);
    my $o = new_ok('MyClass', [dbh => $dbh], 'No exception with handle
+ passed in');
    cmp_ok( $o->dbh->do('select count(*) from db'), '>', 0, 'Found a D
+B via obj');
};

subtest 'With bad params' => sub {
    local $args{user} = 'frobnicator';
    throws_ok { my $o = MyClass->new(dbh => \%args) } qr/coercion for 
+"dbh" failed/;
    throws_ok { my $o = MyClass->new(dbh => \%args) } qr/Access denied
+ for user/;
};

subtest 'With params' => sub {
    my $o = new_ok('MyClass', [dbh => \%args], 'No exception with args
+ hash passed in');
    cmp_ok( $o->dbh->do('select count(*) from db'), '>', 0, 'Found a D
+B via obj');
    dies_ok sub { print $o->dsn }, 'No DSN accessor!';
};

done_testing;

[download]
Output: 
$ prove -lrv 11107494.pl

11107494.pl ..
# Subtest: With no args
    ok 1 - exception on no args
    1..1
ok 1 - With no args
# Subtest: With bad type
    ok 1 - threw Regexp ((?^:Not a DB handle nor a hashref))
    1..1
ok 2 - With bad type
# Subtest: With existing handle
    ok 1 - Found a DB
    ok 2 - 'No exception with handle passed in' isa 'MyClass'
    ok 3 - Found a DB via obj
    1..3
ok 3 - With existing handle
# Subtest: With bad params
    ok 1 - threw Regexp ((?^:coercion for "dbh" failed))
    ok 2 - threw Regexp ((?^:Access denied for user))
    1..2
ok 4 - With bad params
# Subtest: With params
    ok 1 - 'No exception with args hash passed in' isa 'MyClass'
    ok 2 - Found a DB via obj
    ok 3 - No DSN accessor!
    1..3
ok 5 - With params
1..5
ok
All tests successful.
Files=1, Tests=5,  0 wallclock secs ( 0.01 usr  0.00 sys +  0.09 cusr 
+ 0.01 csys =  0.11 CPU)
Result: PASS

[download]

Hope this helps!

update: added test for bad type and moved isa check to coercion


The way forward always starts with a minimal test.

In reply to Re^6: If Perl 5 were to become Perl 7, what (backward-compatible) features would you want to see? by 1nickt
in thread If Perl 5 were to become Perl 7, what (backward-compatible) features would you want to see? by haukex

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