if (-e "/usr/bin/sendmail") {
    $sendmail = "/usr/bin/sendmail";
}#if
if (-e "/usr/sbin/sendmail") {
    $sendmail = "/usr/sbin/sendmail";
}#if
if (-e "/usr/lib/sendmail") {
    $sendmail = "/usr/lib/sendmail";
}#if
if (-e "/var/qmail/bin/qmail-inject") {
    $sendmail = "/var/qmail/bin/qmail-inject";
}#if

##</code><code>##

my $return = `/usr/lib/sendmail --help`;

##</code><code>##

if ( system( '/usr/lib/sendmail -t ...' ) eq 'EX_OK' ) {
    ...
}#if

##</code><code>##

unless ( system( '/usr/lib/sendmail -bv to:test@test.com' ) ) {
    ### OK
}#unless