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
[download]

Alternatively, also using List::Util, but now use first:

use List::Util qw(first);

my @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47);

sub listprimes {
    my $n = shift;
    my @foo;
    first { !($_ < $n ? push @foo, $_ : 0) } @primes;
    @foo;
    }

sub random {
    my ($lo, $hi) = @_;
    return $lo + int rand $hi - $lo + 1;
    }

for (1 .. 5) {
    my $r = random (10, 50);
    my @f = listprimes ($r);
    say "r == $r:  @f";
    }
[download]


Enjoy, Have FUN! H.Merijn

Thanks so much. I am trying to rewrite this program by using the foreach and push function. I think I am not right, can hint me?

#!/usr/bin/perl -w 
use strict; 
my @primes = (2,3,5,7,11,13,17,19,23,29,31,37,41,43,47);


sub listprimes { 
my $n = shift;
my $i = 0; 
my $answer = ""; 

foreach $x ($primes[$i]<=$n) { 
  push $x{$n}
 }
 

 sub random {
 my($a,$b) = @_;
 return int(rand($b-$a+1))+$a;
 } 

my $a = random(10,50);
my $f = listprimes($a);
print "$f\n";
[download]