This version of your code works perfectly (as shown by the output at the end):

#!/usr/bin/perl

#use DBI;
use POSIX;
#use Data::Dumper;
#use Fcntl ':flock';
use strict;
use warnings;
use threads;
use threads::shared;
#use "some IPAM system's package name";

my $host = "1.1.1.1";
my $database = "some_db";
my $user = "some_user";
my $pswd = "some_pswd";

my @threads;

#my $dbh = DBI->connect("DBI:mysql:$database;host=$host", $user, $pswd
+ ,) || die "Could not connect to database: $DBI::errstr";
#$dbh->{'mysql_auto_reconnect'} = 1;

my $sql = qq{
    #a select statement
};

#my $sth = $dbh->prepare($sql);
#$sth->execute();
my $rows = 0;
my @uni_array;

#while(my @request = $sth->fetchrow_array())
for( 1 .. 10 )
{

    #initiated IPAM session


    my @retrieved_objs = ( 1..10 ); #searching for some results

    push @uni_array, @retrieved_objs;

    foreach(@retrieved_objs)
    {
        $rows++;
    }
}

my $number = $rows/10;
my $ceil = ceil($number);

for ( my $count = 1; $count <= $ceil; $count++) {
        my $t = threads->new(\&sub1, $count);
        push(@threads,$t);
}

foreach (@threads) {

        my $num = $_->join();
        print "done with $num\n";
}

print "End of main program\n";

sub sub1 {
        my $num = shift;

        print "started thread $num\n";
        #some code

        sleep($num);
        print "done with thread $num\n";
        return $num;
}
#$sth->finish();
#$dbh->disconnect;
__END__
C:\test>junk28
started thread 1
started thread 2
started thread 3
started thread 4
started thread 5
started thread 6
started thread 7
started thread 8
started thread 9
started thread 10
done with thread 1
done with 1
done with thread 2
done with 2
done with thread 3
done with 3
done with thread 4
done with 4
done with thread 5
done with 5
done with thread 6
done with 6
done with thread 7
done with 7
done with thread 8
done with 8
done with thread 9
done with 9
done with thread 10
done with 10
End of main program
[download]

Which means that the problem lies not with the threading, but with that you've omitted (ie.#some code), and since we are not clairvoyant, we can't really help with that!

(My guess would be that this package: "some IPAM system's package name" is not thread-safe; but it is just a guess.)