I'd use BrowserUk's approach of having a thread pool and feeding jobs to that pool through a queue. Each thread then returns its results to the main program again through a queue.
You can easily turn your serial program into such a queue-based program after you have a working serial program. You haven't made clear that your existing program already works as a single-threaded program, so I can't give you much working code, but given your vague outline above, I'd use something like this:
#!perl -w
use strict;
use threads;
my $THREADS = 20;
my $request = Thread::Queue->new;
my $response = Thread::Queue->new;
my %dataEntity = (
...
);
# Submit all requests
for my $dbname (keys %dataEntity) {
$request->enqueue([ $dbname, $dataEntity{ $dbname } ]);
};
# Tell each thread that we're done
for (1..$THREADS) {
$request->enqueue( undef );
};
# Launch our threads
for (1..$THREADS) {
async(\&getData);
};
sub getData {
while (my $job = $request->dequeue()) {
my ($dbname, $credentials) = @$job;
# connect to DB, retrieve information
$response->enqueue( "Results from $dbname" );
};
# tell our main thread we're done
$response->enqueue( undef );
};
while (my $payload = $response->dequeue or $THREADS--) {
print Dumper $payload;
};