my $mailer = '/usr/sbin/sendmail -t -oi';
my $recipient = 'foo@bar.com';
my $message = '...';

open my $mailer, "|$mailer" or die $!;

print $mailer <<"END" or die $!;
From: user\@domain.com
To: $recipient
Subject: Testing 123

$message
END

close $mailer or die $!;