If you're asking how to write a function that can process either a scalar or a list of values (operating on each value in the list in turn) and the function returns EITHER a scalar OR a list see the Camel book for 'wantarray' function. Here's a quick example for you:

# Trim beginning and trailing whitespace only
sub trim {
  my @out=@_;  # reassign so we don't change input values
  for (@out) {
    # loop and substitution are using default system variable $_
    s/^\s+//;
    s/\s+$//;
  }

  # the return uses Perl's only trinary operator
  #   COND ? THEN : ELSE
  # Was this subroutine called in list context or not?
  return wantarray ? @out : $out[0];
}

# main program
my @list=('  List 1   ','  List 2  ','  List 3  ');
my $val='   Scalar    ';
print "'$_'\n" foreach (trim($val));
print "'$_'\n" foreach (trim(@list));
__END__
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Hope this answers your question.

Even more fun (but less efficient) than this would be to just make the function recursive.



sub add_two
{
    my @in_values = @_;
    if( wantarray )
    {
        #must use +0 to force scalar
        #otherwise we get deep recursion
        return map add_two( $_ ) + 0, @in_values;
    }
    else
    {
        return $in_value[0] + 2;
    }
}
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Thinking as I type, I guess any function that takes one argument and returns one value, *could* return a list if passed a list *and* called in list context. (>1 argument not called in list context might cause a warn/die).

I think this involves the communication skills of a programmer. Who is using this function? Is it it usually called many times or just once or twice. During the past week I was helping a newbie with subs to process partial log files. The sort functions lent themselves to speedy list processing, but users would rarely trigger such calls and we used a loop for multiple calls to increase efficiency of the sub.

It goes back to: "know your customers" or in this case your typical users.