sub foo {
   my ($reset) = @_;
   my $x if $reset;   # Will reset at end of scope if $reset is true.
   return ++$x;
}

print foo(0), "\n";  # 1
print foo(0), "\n";  # 2
print foo(1), "\n";  # 3
print foo(0), "\n";  # 1
print foo(0), "\n";  # 2