Homework? You just need a simple loop.

my @myarray = qw(1 2 3 4 5 6 7 8 sd frd sed sddfsd asecf ssdd myh);
my @fifth;

for (my $i=4; $i < scalar(@myarray); $i += 5)
{
        push(@fifth, $myarray[$i]);
}

print "fifth=".join(",",@fifth)."\n";
[download]