References are not allowed as default values, you must wrap the default of 're' in a CODE reference (ex: sub { [] } and not []) at ...

##</code><code>##

#! perl
use strict;
use warnings;

package Test
{
    use Moose;

    has re => (is => 'rw', isa => 'RegexpRef', default => sub { qr/abc/; });

    sub BUILD
    {
        my $self = shift;

        $self->re(qr/^[0-9]+$/);
    }

    sub match
    {
        my ($self, $string) = @_;

        return $string =~ $self->re ? 'matches' : 'does not match';
    }
}

my $t = Test->new();

print "$_ ", $t->match($_), "\n" for 'abc123def', '456';