sub trim {
    @_ = @_ ? @_ : $_ if defined wantarray;
    # When the caller is looking for a return value 
    # (not void context), make a copy of the arguments, 
    # so the original value is not changed.
    # If there are no arguments passed, 
    # copy the value of $_ instead.

    for (@_ ? @_ : $_) { next unless defined; s/\A\s+//; s/\s+\z// }
    # If @_ contains any values, perform the 
    # substitution on all defined values. If @_ contains 
    # no values, perform the substitution on $_ instead.

    return @_ if wantarray || !defined wantarray;
    # return the changed values in array context 
    # or (for a reason which escapes me) void context.

    if (my @def = grep defined, @_) { 
       return "@def" 
    } else { return }
    # In scalar context, return the results of 
    # the substitution on all
    # defined values, if any. Otherwise just return.

}

##</code><code>##

@trimmed = trim(@not_trimmed);

##</code><code>##

foreach (@untrimmed) {
   say trim;
}
##</code><code>##

sub trim (_;@) { 
   # ... as above
}

my @data = (' a', ' b ' , 'c ');

say trim (@data);