Store the subject and message in variables based on the conditions prior to sending the email.

my ($subject, $message);
if ($old ne $curr) {
  $subject = 'Subject A';
  $message = 'Subject A message here';
} elsif ($var eq $job) {
  $subject = 'Subject B';
  $message = 'Subject B message here';
} else {
  $subject = 'Subject C';
  $message = 'Subject C message here';
}

my %mail = ( To      => 'mail@there.com'',
             From    => 'from@there.com',
             Subject => $subject,
             Message => $message,
           );
$mail{smtp} = $mailserv;
sendmail(%mail) || die "\nProblem! $Mail::Sendmail::error\n";
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One option may be to move all the %mail declarations into an array-of-hash-references or a hash-of-hash references. Then, your if-elsif-else can be converted into a CASE or SWITCH type block which sets a reference to the hash you want to send to sendmail(). This will make your script a little easier to read and make it easier to add items to the CASE/SWITCH by adding more logic and additional entries into the hash.

OTOH, as someone once said: If it ain't baroque, don't fix it. Not to be facetious, but is this script having performance or resource problems? Security issues? Limited features? I understand the need for efficiency and I respect slick coding as much as the next monk, but unless you have a good reason, why modify something that you have been using for a year(in a production environment I assume) unless you want to expand it's functionality or correct a flaw? Just a thought.

Later

my %mail = (
        To   => 'mail@there.com',
        From => 'from@there.com'
    );

if($old ne $curr){
    $mail{'Subject'} = "Subject A";
    $mail{'Message'} = "Subject A message here";
}
elsif($var eq $job){
    $mail{'Subject'} = "Subject B";
    $mail{'Message'} = "Subject B message here";
} else {
    $mail{'Subject'} = "Subject C";
    $mail{'Message'} = "subject C message here";
}

$mail{smtp} = $mailserv;

sendmail(%mail);
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Thank you to all for your quick responses and suggestions!

use strict;
use MIME::Lite;

my $old           = 'Foo';
my $curr          = 'Foo';
my $from          = 'from@there.com';
my $to            = 'mail@there.com';
my $subject       = 'Subject C message here';

# You could nest one more ternary in here for
# '$var eq $job'
$old ne $curr 
       ? $subject = 'Subject A message here' 
       : $subject = 'Subject B message here';

my $msg = MIME::Lite->new( 
          From    => $from,
          To      => $to,
          Subject => $subject,
          Data    => "Here's the message body you wanted",
          Type    => 'text/plain'
);

# You'd want $msg->send; though, of course
print $msg->as_string;
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