Or, if you really are against using globals, use the $parser->{pad} to hold your data:

my @rules = (
  AM_Key => sub { push @{$_[4]->{pad}{am_keys}}, $_[1]{_content}; retu
+rn },
  LK_Key => sub { push @{$_[4]->{pad}{lk_keys}}, $_[1]{_content}; retu
+rn },
  Outpatient_Services => sub { LK_Keys => $_[4]->{pad}{lk_keys}, AM_Ke
+ys => $_[4]->{pad}{am_keys}},
  _default => undef,
);
my $xr = XML::Rules->new( rules => \@rules );
[download]

With this $xr->parse() returns a HoA containing the array of the AM_Keys and the array of LK_Keys.