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

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

...
  my ($href) = @_;              # Makes a COPY of the reference
  $href= {k1=>'v1', k2=>'v2}; # Notice - using CURLY brackets, not par
+ens. This creates a Hash ref.
}