use Modern::Perl;$_='
<use><qd>action</></>
<use><qd>action</></>
';

$_{'s'} = { jeopardy =>"21"};
$_{'qd'} = { action => 'default' };

sub dbl { say "this should only calc once";
          $_{'dbl'}->{$_[0]} = $_[0]*2; }

sub use { $_{'use'}->{$_[0]} = &simulate_opening_file; }

sub simulate_opening_file
   { say "this should only slurp once";
    "The answer is <dbl><s>jeopardy</></>" };

while (/<([^<]+?)>([^<]*?)<\/>/)
     {
      my $l=$+[0]-$-[0];
      no strict 'refs';
      my $r = $_{$1}->{$2} ||= $1->($2);
      substr($_,$-[0],$l,$r);
     }

say;