Thanks for the BEGIN solution; that did the trick. Actual code uses something like

my $case = new Case1(...); 
... 
print $case->xml;
Package Guts;
sub new { ... }
sub xml { return '...'; }
Package Case1;
BEGIN { @Case1::ISA = qw(Guts) }
sub xml {
  my $self = shift;
  return '<case1>' . $self->SUPER::xml . '</case1>';
}
Package Case2;
BEGIN { @Case2::ISA = qw(Guts) }
sub xml {
  my $self = shift;
  return '<case2>' . $self->SUPER::xml . '</case2>';
}
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