use v5.14;
use Data::Dumper;

package MyClass {
	use Moo;
	has base_file => (
		is       => 'rw',
		required => 1,
		init_arg => 'base',
		builder  => sub { return 'bar' },
	);
}

# builder sub is *NOT* executed
my $object1 = MyClass2->new(base => "foo");

print Dumper($object->base_file);  # $VAR1 = 'foo'

# builder sub *IS* executed
my $object2 = MyClass->new();

print Dumper($object2->base_file);  # $VAR1 = 'bar'

##</code><code>##

use v5.14;
use Data::Dumper;
use IO::File;

package MyClass {
	use Moo;
	has base_file => (
		is       => 'rw',
		required => 1,
		init_arg => 'base',
		coerce   => sub {
			my $base_fh = IO::File->new( $_[0], '<' ) or die "$_[0]: $!";
			$base_fh->binmode(":utf8");
			return $base_fh;
		},
	);
}

my $object = MyClass->new(base => __FILE__);
print Dumper($object->base_file);

##</code><code>##

use v5.14;
use Data::Dumper;
use IO::File;

package MyClass {
	use Moo;
	has base    => (
		is       => 'rw',
		required => 1,
	);
	has base_file => (
		is       => 'rw',
		lazy     => 1,
		builder  => sub {
			my $base_fh = IO::File->new($_[0]->base, '<' ) or die($_[0]->base . ": $!");
			$base_fh->binmode(":utf8");
			return $base_fh;
		},
	);
}

my $object = MyClass->new(base => __FILE__);
print Dumper($object->base_file);