...  
$sth->execute;
my (%rows,%cols,$rtime,$dev,$value);
while (($rtime,$dev,$value)=$sth->fetchrow) {
   $rows{$rtime}{$dev}=$value;
   $cols{$dev}++;
}
$sth->finish if $sth->{Active};  # really don't need finish if you got all the data
# now column headers can be computed
my @cols=sort keys %cols;
map { $cols{$cols[$_]}=$_+1 } 0..$#cols;  # get indexes
my @headers=('RTIME',sort keys %cols);
my @row;
print join("\t",@headers),"\n";
# and rows can be extracted (empty cells where a dev wasn't seen for a particular rtime)
for $rtime (sort keys %rows) {
   $row[0]=$rtime;
   map { $row[$cols{$_}]=$rows{$rtime}{$_} } keys %{$rows{$rtime}}; 
   print join("\t",@row),"\n";
}