my $vergleich = 4;
my @a = (3,4,14,43);
my $isin = 0;
foreach my $elem (@a) {
    if ($vergleich == $elem) {
        $isin = 1;
        last;
    }
}
if ($isin) {
    ... mache planA ...
} else {
    ... mache planB ...
}