my $stats = [];
my $code  = create_closure( $stats );

....

$pm->run_on_finish( $code );

....

print Dumper( $stats );


sub create_closure {
  my( $stats ) = @_;
  return sub {
    my ($pid, $exit_code, $ident) = @_;
    push( @$stats, { 
      pid       => $pid, 
      exit_code => $exit_code, 
      ident => $ident 
    } );
  }
}