Other monks have already pointed you in the right direction with passing structures by reference.

I just want to point out that it is possible not to pass by reference for your benefit.

A simple hash basically has keys and corresponding values. Knowning this, you can write your code like this:

whatever(%hash, $val)
...
sub whatever
{
  my $v = pop;
  my %h = @_;  # %hash was expanded into list of key-value pairs
               # reconstruct hash from array of key-value pairs
  ...
}
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