I would have gone this way:

$option_line = join ' ', @ARGV; #reconstitute
my @pairs = $option_line =~ m/(\w+:\s+\w+)/g; # grab option pairs
my %option;
for $pair (@pairs)
{
    # now split
    my ($name,$value) = split ":\s+", $pair, 2;
    # save each pair in hash
    $option{$name} = $value;
}
[download]

Or you could use G::D, but this might be overkill:

our %opt_hash; #can't use my
use Getopt::Declare;
my $spec = q{
    <name>: <value>    Name/Value pair
    { $::opt_hash{$name} = $value; }
    [repeatable]
};
# untested
[download]

(I can't recall if whitespace is automagically optional, and I have to run %/;)

Update: Missing $option_line corrected.