Simplistically:

#! perl -slw
use strict;
use threads;
use Thread::Queue;
use DBI;


our $N //= 4;

sub child {
    my $Q = shift;
    while( my $work = $Q->dequeue ) {
        for my $thing ( @{ $work } ) {
            ## process data items
        }
    }
}

my $Q = new Thread::Queue;

my @kids = map threads->create( \&child, $Q ), 1 .. $N;

my $dbh = DBI->connect( ... );
my $sth->prepare( ...SQL HERE... );

while( my $ref = sth->fetch_* ) {
    $Q->enqueue( share_clone( $ref ) );
}

### Assume exiting the loop above means no more data
$dbh->disconnect;

## tell the kids to die
$Q->enqueue( ( undef ) x $N );

## And wait for kids to clean up
$_->join for @kids;;
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