use strict 'refs'; is what's preventing simulate_opening_file from being called if someone did <simulate_opening_file>...</>, so I applaud your desire to use it.

Two options:

• Keep using no strict 'refs';, but require that the callbacks are located in a specific patckage that contains nothing but callbacks.

  1 while s/<([^<]+?)>([^<]*?)<\/>/
     $_{$1}->{$2} ||= do {
        #TODO: Make sure $1 doesn't contains "::".
        my $func = $pkg . "::" . $1;
        #TODO: Make sure $func exists.
        no strict 'refs';
        $func->($2)
     }
  /e;
  [download]

• Require that callbacks get registered.

  my %cb = (
     use => \&use,
     dbl => \&dbl,
  );
  
  1 while s/<([^<]+?)>([^<]*?)<\/>/
     #TODO: Make sure $cb->{$1} exists.
     $_{$1}->{$2} ||= $cb->{$1}->($2)
  /e;
  [download]

By the way, you'll have problems if the replacement value contains a "<".