I might implement that something like this:

%SORT_INFO = (
   'firstname'     => sub { $a->{FirstName} cmp $b->{FirstName} },
   'firstname_desc => sub { $b->{FirstName} cmp $a->{FirstName} },
   ...
);

...

if (exists $SORT_INFO{param('sort_by')}) {
   @sorted = sort $SORT_INFO{param('sort_by')} @unsorted;

} elsif (param('sort_by')) {
   die "Unknown sort_by item: " . param('sort_by');

} else {
   @sorted = @unsorted;
}
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