The combination of join and grep sounds like it will do what you want. (map and sort and some of their friends from List::Util and List::MoreUtils are also often useful, but don't sound needed in your case.)

The following example uses join and grep to filter the numbers from 1 to 20 for primes, then join the primes together into a string separated by semicolons.

sub is_prime
{
   my $num = shift;
   return if $num == 1;  # 1 is not prime
   
   die "usage: is_prime(NATURAL NUMBER)"
      unless $num =~ /^[1-9][0-9]*$/;
   
   for my $div (2 .. sqrt $num)
   {
      return if $num % $div == 0;
   }
   return 1;
}

my @numbers = 1 .. 20;

print join ";", grep { is_prime($_) } @numbers;
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