Depends on what you want it to do. You need to be aware that:

• It won't work if it's in a module that you're importing elsewhere (because you end up changing Module::foo, which is irrelevant to the ModuleUser::foo that was imported).

  You can change that with

    no strict 'refs';
    my $pkg = caller;
    *{ $pkg . "::foo" } = sub { ... }; # the final foo
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  but only if that's what you want ("call once per user" instead of "call once globally").
• There's no easy way to reset the trigger after it's fired. For your application, this is probably what you want, but you could imagine scenarios where it is not (eg mod_perl).

ikegami's lexical variable version gives "call once globally" semantics, without allowing resets (which you can trivially add by introducing another subroutine that captures the same lexical.) Same for the global variable, except resetting can be done directly.

Personally, I like the idiom

sub foo {
  our $CALL_COUNT;
  call_me_only_once() unless $CALL_COUNT++;
  ...do stuff...
}
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But I'll admit I always wonder whether the code is going to get run 4.3 billion times and redo the init code...