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my @a = qw( a b );
my @b = map { my $x = $_;
              map { $x . $_ } @a;
            } @a;

my @a = qw( a b c );

my @b = map { my $x = $_;
              map { my $y = $_;
                    map { $x . $y . $_ } @a;
                  } @a;
            } @a;

sub f
{ my ( $n, @a ) = @_;
...
        map { $x . $_ ) } f( $n - 1, @a );
      } @a;
}

sub f
{ my ( $n, $g, @a ) = @_;
...
        map { $g->( $x, $_ ) } f( $n - 1, $g, @a );
      } @a;
}