use vars '$have_Storable';
$have_Storable = do {
    eval {
        require Storable;
        import Storable 'dclone';
    };
    $@ ? 0 : 1;
};

# ...
# code predicated on the setting of $have_Storable

##</code><code>##

{
    local $Regexp::Assemble::have_Storable = 0;
    my $orig = Regexp::Assemble->new->add( qw/ dig dug dog / );
    my $clone = $orig->clone;
    is_deeply( $orig, $clone, 'clone path' );
}