my %hash;
populate_hash(%hash);

sub populate_hash{
  my %h = @_;              # Makes a COPY of the paramter
  %h= (k1=>'v1', k2=>'v2); # Populates the LOCAL %h, not '%hash'
}

##</code><code>##

 my %hash = populate_hash( ); # Option 1: Use the Return value

 # Option 2: pass a REFERENCE to '%hash', like this:
 populate_hash(\%hash);
  # This requires "populate_hash" to be aware of, and use the reference, like this:
sub populate_hash{
  my ($href) = @_;              # Makes a COPY of the reference
  $href= {k1=>'v1', k2=>'v2}; # Notice - using CURLY brackets, not parens. This creates a Hash ref.
}