sub f {
   my ($x) = @_;
   '' =~ /(?{ print "$x\n" })/;
}

f(4);  # 4
f(5);  # 4!!

##</code><code>##

sub f {
   my ($x) = @_;
   $block ||= sub { print "$x\n" };
   $block->();
}

f(4);  # 4
f(5);  # 4!!

##</code><code>##

BEGIN {
   package Prefixer;

   sub new {
      my ($class, $prefix) = @_;
      return bless({ prefix => $prefix }, $class);
   }

   sub prefix {
      my ($self) = @_;
      return join '', $self->{prefix}, @_;
   }
}

my $a_prefixer = Prefixer->new('a');
my $b_prefixer = Prefixer->new('b');
print("$_\n")
   for $a_prefixer->prefix('1'),  # a1
       $a_prefixer->prefix('2'),  # a2
       $b_prefixer->prefix('3'),  # b3
       $b_prefixer->prefix('4');  # b4

##</code><code>##

sub make_prefixer {
   my ($prefix) = @_;
   return sub { return join '', $prefix, @_ };
}

my $a_prefixer = make_prefixer('a');
my $b_prefixer = make_prefixer('b');
print("$_\n")
   for $a_prefixer->('1'),  # a1
       $a_prefixer->('2'),  # a2
       $b_prefixer->('3'),  # b3
       $b_prefixer->('4');  # b4

##</code><code>##

sub f {
   local our ($x) = @_;
   $block ||= sub { print "$x\n" };
   $block->();
}

f(4);  # 4
f(5);  # 5

##</code><code>##

sub f {
   local our ($x) = @_;
   '' =~ /(?{ print "$x\n" })/;
}

f(4);  # 4
f(5);  # 5