If you want you can use Net::Telnet. Check the port and see if you get a 2xx back from the server.

If the request was sucessful each line should be prepended by a 2xx status code and SMTP in the line.

$line =~ /^2\d\d .*SMTP/ should work as a rudimentary test for the first line back.

Update:

use strict;
use warnings;

use Net::Telnet;

my $tel = Net::Telnet->new(-host => 'localhost', -port => 25, -errmode
+ => 'return') 
     or die "Port not available\n";
my $rtn = $tel->getline() 
     or die "No Reponse\n";
print $rtn =~ /^2\d\d .*SMTP/ ? 'OK' : 'Fail';
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