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