The easiest way is to use an anonymous subroutine to call your method:

# i'm assuming $self is actually defined here somewhere...
    my $twig = new XML::Twig(
        twig_handlers => {
            _all_ => sub { $self->proc_list(@_) },
        },
    );
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addendum:

$ele is always empty inside proc_list.

That's because in your code you're not passing the proc_list method, you're passing the result of calling $self->proc_list; which 1) is called before the XML::Twig object is even constructed, and 2) does not pass any arguments beside $self to proc_list.