There is no need for a prototype which also means you don't need to forgo the OO if you don't want to.

sub int_monitor:lvalue {
  my $var=\shift;
  print "$var\n";
  $$var
};
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Very neat! I guess that relies on two facts: that @_ contains aliases to the args (which I knew but had forgotten) and that shift returns the first element of the array, rather than a copy of it.

Thanks for your suggestions,

Markus