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.