Consider the following:

sub find_above_average {
    my $mean = &calculate_average(@_);

    return grep $_ > $mean, @_;
}
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Remove the say line and print the average outside of the subroutine, and use grep to return only those values greater than the value of $mean.

sub calculate_average {

    @_ > 0 or die 'No numbers sent for averaging.';

    my $sum = 0;
    foreach (@_) {
        $sum += $_;
    }

    return $sum / @_;
}
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Check for an empty list at the top to avoid a division by zero error.