&$ref_Gen_HTML is a (special form of) subroutine call. You want to pass the reference, $ref_Gen_HTML.

my $ref_Gen_HTML = sub { $self->Gen_HTML() }; 
print $pjx->build_html( $self->{$WU_CGI}, $ref_Gen_HTML );
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or just

print $pjx->build_html( $self->{$WU_CGI}, sub { $self->Gen_HTML() } );
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Please use <c>...</c> instead of <pre>...</pre> on PerlMonks.