Q. How do I pass a hash reference to a subroutine in Perl?

A. This can be achieved by preceeding the hash with a slash when passing:

#assume you have a hash
my %results = (start_date => "may 1", end_date => "sep 1");

#pass the hash
test(\%results);

#your subroutine
sub test {
my $ref = shift;

# assign new variable
$ref->{'start_date'} = "new date";

}