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