my @hours; # so we have a permanent variable with our data
while (my @row = $sth->fetchrow_array) { push(@hours, $_) for @row }

my %byhour;
foreach my $hours (@hours)
{
  $hours =~ /\s(\d\d)/; 
  my $knownhour = $1;

  ##########
  # enter hours into our hash for counting
  ##########
  if (exists $byhour{$knownhour})
  {
     my $hourcount = $byhour{$knownhour};
     $hourcount++;
     $byhour{$knownhour} = $hourcount;
  }
  else
  {
     $byhour{$knownhour} = 1;
  }
}

foreach (keys %byhour) { print "$_ had $byhour{$_}<br>"; }