Me too :)

#!/usr/bin/perl --
use strict;
use warnings;
use Data::Dump;
use IO::File;

Main( @ARGV );
exit( 0 );

BEGIN
{
    package Shack;
    use Moo;
    has(
        qw/
            crabs
            is rw
            required 1
            init_arg base
        /,
        builder => sub {
            warn "## BUILDER { @_ } @{[Data::Dump::pp( @_ )]}\n";
            IO::File->new( $_[0] ) or die qq{"$_[0]": $!\n$^E\n };
        },
    );
    $INC{'Shack.pm'} = __FILE__;
}

sub Main {
    use Shack;
    dd( Shack->new( base => __FILE__ ) );
    dd( Shack->new( crabs => __FILE__ ) );
}

__END__
bless({ crabs => "crabshack.pl" }, "Shack")
## BUILDER { Shack=HASH(0xc3f7ec) } bless({}, "Shack")
"Shack=HASH(0xc3f7ec)": No such file or directory
The system cannot find the file specified
  at crabshack.pl line 23.
[download]

So BUILDER doesn't get called if there is a base argument (named in init_arg

I think the idea is to have

has qw/ base required 1 /;
has qw/ crabs is rw / , builder => sub { IO::File->new( $self->base ) 
+or  ... };
[download]

But I don't get it, even after looking at
https://metacpan.org/source/HAARG/Moo-1.003000/t/init-arg.t
https://metacpan.org/source/HAARG/Moo-1.003000/t/lazy_isa.t

No, I'm not familiar with Moose way of doing OOP, maybe the Moose docs explain the concepts better so the Moo docs make sense, but I'm not that interested