Very nice. As always, it should be noted that relying on the stringification of references may lead to surprises if overloading has been brought into play. Also, with a more specific pattern:

package WWW::Mechanize::NoRefs;
use base 'WWW::Mechanize';

my $rx = qr/(
    (?: [A-Za-z]\w* (?: :: [A-Za-z]\w* )* = )?
    (?: SCALAR | ARRAY | HASH | CODE | REF | GLOB | LVALUE )
    \( 0x [a-fA-F0-9]+ \)
)/x;

sub content {
    my $self    = shift;
    my $content = $self->SUPER::content;
    if ($content =~ $rx) {
        require Carp;
        Carp::carp("Possible stringified reference ($1) found in conte
+nt");
    }
    return $content;
}

1;
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