OK; I think you're confused about what builders are supposed to do. Your builder will only be executed if you do NOT provide a value for the base_file attribute.

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'

[download]

Further, you seem to be assuming that within the builder sub, $_[0] is some sort of file name. It's not; it's $self.

I think what you want is coercions...

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);

[download]

In summary: use coercions to munge incoming values; use builders to provide a default value when there is no incoming value.

Here's another way you could have done it... provide a base attribute which is the filename as a string, and then build the base_file attribute from that:

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);

[download]
package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name

In reply to Re: Confusion with Moo's builder attribute and a file handle by tobyink
in thread Confusion with Moo's builder attribute and a file handle by mgatto

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