return join " ", grep {$_ <= $n} @primes;

This use of grep will iterate over the entire list of  @primes even after the  $n limit is encountered. This is not a problem for a list of the first fifteen primes, but might be for the first 15 million primes! One way to mitigate this overhead would be with the List::MoreUtils::last_index() function: 

c:\@Work\Perl\monks>perl -le
"use warnings;
 use strict;
 ;;
 use List::MoreUtils qw(last_index);
 ;;
 my @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47)
+;
 print qq{primes:   @primes};
 ;;
 sub listprimes {
   my ($n) = @_;
   ;;
   my $last_i = last_index { $_ <= $n } @primes;
   ;;
   return join ' ', @primes[ 0 .. $last_i ];
   }
 ;;
 sub random {
   my ($lo, $hi) = @_;
   return $lo + int rand $hi - $lo + 1;
   }
 ;;
 for (1 .. 5) {
   my $r = random(10, 50);
   my $f = listprimes($r);
   print qq{r == $r:  $f};
   }
"
primes:   2 3 5 7 11 13 17 19 23 29 31 37 41 43 47
r == 50:  2 3 5 7 11 13 17 19 23 29 31 37 41 43 47
r == 32:  2 3 5 7 11 13 17 19 23 29 31
r == 18:  2 3 5 7 11 13 17
r == 47:  2 3 5 7 11 13 17 19 23 29 31 37 41 43 47
r == 25:  2 3 5 7 11 13 17 19 23

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In reply to Re^2: Perl program - I hope I can more understand my code by AnomalousMonk
in thread Perl program - I hope I can more understand my code by brianphan

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