It probably stands to reason you should re-think your design, because this looks highly unusual.

If you are determined to make it work, then this should fit the bill:

$mw->Button(command => sub { $self->func1($a) })->pack();

Instead of using a subroutine reference, this uses an anonymous subroutine which establishes the correct object and parameters. Note, however, that if $self changes, the subroutine is a closure and might not have the same value as you'd expect. Test carefully.