my @one = qw( 1 3 5 7 9 11 13 15 17 19 21 23 );
my @two = qw( 1   5   9    13    17    21    );

my %valid; 

@valid{@one} = (1) x @one; 
@valid{@two} = (0) x @two; 

my @choices = keys %valid; 

sub randelt { $choices[rand @choices] }