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.