For info, the actual program in which this logic appears does this:

Now that would be interesting to see...

But for this specific problem, I modified your thread code as follows:

sub RunThread {
    my $tid = threads->tid;
    my $dbh = DBI->connect("DBD::Pg",...) or die "failed connect";
    my ($x, $y)=@_;
    open (my $fh2, ">/tmp/da-$x-$y") or die "failed in thread $! at it
+er $x thread $y " ;
    print "$tid : ", fileno( $fh2 ); sleep 1;
    close ($fh2);
    $dbh->disconnect or die "disconnect failed";
}
[download]

When I run that, I get the following which shows that the filenos underlaying the file handles are being reused by each new batch of threads. If you don't see similar, then it probably means that Solaris/Perl on your system is downlevel or broken.

[19:04:06.98] c:\test>DBjunk.pl
1 : 3
2 : 4
3 : 5
4 : 6
5 : 7
6 : 8
8 : 9
10 : 10
9 : 11
7 : 12
12 : 13
14 : 14
13 : 15
11 : 16
15 : 17
16 : 3
17 : 4
19 : 5
18 : 6
20 : 7
21 : 8
22 : 9
23 : 10
24 : 11
25 : 12
27 : 3
28 : 4
26 : 5
29 : 6
30 : 7
31 : 8
32 : 3
33 : 4
34 : 5
35 : 6
36 : 7
37 : 8
38 : 9
39 : 10
40 : 11
41 : 12
[download]