Getopt::Long in this case is just letting you be flexible -- the '-' is a valid character in a string, so it lets you put it in there. What I've done in the past is set up a collection of regular expressions to validate the strings passed in. You can do this after argument processing, or during using anonymous subs.

This ain't pretty, but it'll work.

my %script_opts = ();

GetOptions("foo=s" => sub { if ($_[1] =~ /^([^-]).*$/) {
    $script_opts{$_[0]} = $_[1];}  }
);
[download]

Documentation for the module is here.