It's kinda hard to check your code without some data to drive match_query. However if $count doesn't get any entries the assignment my $max_count = shift @array; will assign undef and give the warning described.

Note that your code is very Cish making it harder than it needs to be to see what is going on. Replace your C for loops with Perl for loops over the range 0 .. length($key)-1 for a start. You should also use %count and %element so it's obvious what type is being used instead of depending on some Perl magic to create an appropriate reference at run time. That makes the code easier to read and provides better error checking.

Don't use prototype subs (sub match_query() {). They don't do what you want and in the case of match_query your prototype is clearly wrong. The sub says don't give me any arguments in the prototype, then shifts off two arguments!

If the hint in the first paragraph isn't enough you'll need to mock up something to call match_query with appropriate arguments that we can run ourselves.