use warnings;
use strict;

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

sub random_from
{
        my ($first, $second) = @_;
        my %invalid = (map { $_ => 1 } @$second);
        my $return;

        do {
                # Choose a random key from the combined listing
                $return = $first->[rand(@$first)];
        } while ($invalid{$return});

        return $return;
}

print "Selection: ",random_from(\@first, \@second),$/;