my @array;
my ($a, $b, $c);
$a = "foo";
$c = "bar";

if ($a) {            # could also be written "if ( defined($a) ) {"
    push @array, $a;
} else {
    $a = "NA";
    push @array, $a;
}

if ($b) {
    push @array, $b;
} else {
    $b = "NA";
    push @array, $b;
}

if ($c) {
    push @array, $c;
} else {
    $c = "NA";
    push @array, $c;
}

print Dumper @array;