Can['t] use the exists() function either, because it's use on arrays is deprecated as well.

It wouldn't help any way. exists on an array only tests if the array element at a given index exists (ie. has not been deleted), and has nothing to do with what it contains.

Is there a proper, future-proof alternative that does not require me to sprinkle foreach test loops all over the place?

Like this:

if( grep /^\Q$scalar$/, @array ) {
    ## it's in there
}
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Of course, that uses the implicit $_; so perhaps you'd prefer:

if( grep $_ eq $scalar, @array ) {
    ## it's in there
}
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