Looks like part of URL. You can parse it using split or regex, or use CGI::Simple:

use warnings;
use strict;

use CGI::Simple;

my $query = '&keyname=value&keyname2=value2&keyname2=value3';
my $c = CGI::Simple->new($query);
for my $p ($c->param) {
  printf "%s: %s\n", $p, join(',', $c->param($p));
}

__END__

keyname: value
keyname2: value2,value3
[download]

You could do something like this:

my $string = '&keyname=value&keyname2=value2';

# eliminate the first ampersand for cleaner output
$string =~ s/^&//; 

foreach ( split /&/, $string ) {
    printf "Key: %s, Value: %s\n", split /=/;
}
[download]

I'd also look on CPAN for something that parses URL queries.