$var = $var || 30;
# or
$var ||= 30;
# or
$var = $var ? $var : 30;
# or if you care about definedness rather than Boolean truth:
$var = $var // 30;
# or
#var //= 30;
# or
$var = defined($var) ? $var : 30;

##</code><code>##

if (! defined $var) {
    $x = 40;
}
else {
    $x = 50;
}

##</code><code>##

$x = defined $var ? 50 : 40;