So if I have this right, @_ holds the list of parameters passed to the abv subroutine.

You got it, here's a version that uses the special variables $_ and @_

sub abv {
    my $avg = avg(@_);
    my @cands = ();
    foreach (@_) {
        ($_ > $avg) and push(@cands, $_);
    }
    return @cands;
}
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I flicked the if statement into a logical and, for each iteration of the foreach $_ will be the same as $cand was in the original function.