listprimes() can be re-written even more efficiently using the Foreach loop.

Push is not necessary to achieve the result, but here is how you could use it:

 sub listprimes {
    my ($n) = @_;
    my @primes_leq_n;
  
    for my $p (@primes){ # for and foreach are synonymous
       last if $p > $n;
       push @primes_leq_n, $p;
    }
    return join " ", @primes_leq_n;
 }
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