$MyApp::once{x} ||= 1;

##</code><code>##

sub f {
  ...
  log('some information'); # but should be called only the first time.
  ...
}

##</code><code>##

  static LogRetType const once=log('some information');

##</code><code>##

my $once1=0;
sub f {
  ...
  $once1 ||= (log('some information'),1);
  ...
}

##</code><code>##

sub f {
   ...
   $MyApp{+__FILE__.' '.__LINE__} ||= (log('some information'),1);
   ...
}