Update

I like both the replies that just came in. Thank you both!

I am given a configuration hash reference and three keys of interest. I have to return a hash reference that contains at most the three keys of interest, but omits them if their configured value is false, undefined, or nonexistent. So the simple hash-slicing solution 

my %small_hash;
@small_hash{@interesting_keys} 
    = @$config{@interesting_keys};
return \%small_hash;

[download]

is not acceptable, because keys of interest with false or undefined values are retained, and I want them not to exist.

I have written the following pedestrian solution: 

# $config is given

sub get_file_watcher_ref {
    my %setting_for;
    my (
        $watch_interval,
        $watch_file_min_size,
        $term_run_time,
    );
    
    if ( $watch_interval = $config->{watch_interval} ) {
        $setting_for{watch_interval} = $watch_interval;
    }
    if ( $watch_file_min_size = $config->{watch_file_min_size} ) {
        $setting_for{watch_file_min_size} = $watch_file_min_size;
    }
    if ( $term_run_time = $config->{term_run_time} ) {
        $setting_for{term_run_time} = $term_run_time;
    }

    return \%setting_for;
}

[download]

which (barring typos - I haven't tested it yet) does what I want done. But it's horribly repetitive.

Does anyone know an elegant way to write this?

Update

I must have momentarily forgotten how to loop. Here's what I meant to do. 

sub get_file_watcher_ref {
    my %setting_for;
    my @interesting_keys = qw(
        watch_interval
        watch_file_min_size
        term_run_time
    );
    
    for (@interesting_keys) {
        if ($config->{$_}) $setting_for{$_} = $config->{$_};
    }
    
    return \%setting_for;
}

[download]

The loop still feels kind of clumsy. Any suggestions for improvement?

In reply to [Answered!] Taking a subset of a hash by Narveson

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