As it seems that your mail server is exim, you can send the messages to it through a pipe using "batch SMTP":
open my $exim, "| exim -bS" or die;
for (...) {
print $exim <<'MSG';
MAIL FROM: foo@bar.com
RCPT TO: doz@bar.com
DATA
Subject: Your Invoice
To: You
From: Me
You have to pay 1,000,000 dollars
.
MSG
}
If you want to parallelize, fork some processes and divide the task between them:
my $workers = 4;
for my $ix (0..$workers-1) {
unless (fork) {
open my $fh, '<', $address_list_file_name or die;
open my $exim, "| exim -bS" or die;
while (<>) {
next unless (($. % $workers) == $ix);
send_invoice($exim, $_);
}
exit 0;
}
}