my $string1 = "one=1,two=2,three=3,four=4,five=5";
my $string2 = "one=1,three=3,five=5";

$string1 =~ m/
                (?:one=(.*?),)?
                .*
                (?:two=(.*?),)?
                .*
                (?:three=(.*?),)?
                .*
                (?:four=(.*?),)?
                .*
                (?:five=(.*?),)?
        /x;
print "one: $1, two: $2, three: $3, four: $4, five: $5\n";