Straightforward solution:

my $matches = 2;
my $threshold = 8;

my @results = ();

foreach my $row (@arr) {
  my $count;
  for (1..$#$row) {
    $count++ if($row->[$_] >= $threshold);
  }
  push @results, $row if($count >= $matches);
}

foreach(@results) {
  say join ",", @$_;
}
[download]

Shorter/idiomatic/obfuscated (take your pick) solution:

my $matches = 2;
my $threshold = 8;

my @results = grep {
  $matches <= scalar grep { $_ >= $threshold } @$_[1..$#$_];
} @arr;

foreach(@results) {
  say join ",", @$_;
}
[download]

Both of these output:

D,6,7,8,8
F,4,8,8,8
[download]

BTW, I'm assuming here that you're actually interested in whether there's at least $matches item equal to or greater than $threshold, since that's what "threshold" implies. If you're only interest in exactly that value, change >= $threshold to == $threshold in either code snippet above.