sub crossMul{ 
    my( $r, $first ) = ( shift, shift ); 
    print "$first * $_ = ", $r->{$first} * $r->{$_} for @_; 
    crossMul( $r, @_ ) if @_ > 1; 
};;

%hash=( A=>4, B=>5, C=>10, D=>2, E=>9 );;

crossMul( \%hash, sort keys %hash );;
A * B =  20
A * C =  40
A * D =  8
A * E =  36
B * C =  50
B * D =  10
B * E =  45
C * D =  20
C * E =  90
D * E =  18