update: something like that would do:

...
sub import {
    shift;
    $config or $config = XMLin(shift) if @_;
    $XSConf::strict    = shift        if @_;
    return; # don't give $config away
}

....
            ref $thing and $token = $thing or return bless \$thing, XS
+Conf::Str;
        }
    }
    return $package;
}

package XSConf::Str;

use overload '""' => \&str, fallback => 1;

sub str { ${$_[0]} }

our $AUTOLOAD;

sub AUTOLOAD {
    return if $AUTOLOAD eq 'XSConf::Str::DESTROY';
    my @caller = caller;
    die "you cannot invoke the method '$AUTOLOAD' on the string '$_[0]
+'"
     . " at $caller[1] line $caller[2]\n" if $XSConf::strict;
    $_[0];
}   
1;
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update: added $XSConf::strict, caller and die