#!/usr/bin/perl

use strict;
use warnings;

my %h = qw( a b c d );
func( \%h );

while ( my ( $key, $val ) = each %h ) {
  print "key is $key val is $val\n";
}

sub func {

  my $incoming_hash_ref = shift;

  $incoming_hash_ref->{'e'} = 'f';
}
[download]

prints:

key is e val is f
key is c val is d
key is a val is b
[download]

IOW, the $_ isn't doing what you think it is doing (AFAICT).

Steve