What you're looking for is %{$Ref_to_Hash}:

sub Modify_Hash_1 {
  my ( $Ref_to_Hash ) = @_;
  # Note, _NOT_ {} - must be assigning a hash,
  # not a hash ref
  %{$Ref_to_Hash} = ( "Pears", 5, "Peaches", 7 );
};
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That way, you're saying "update the hash pointed to by $Ref_to_Hash".

I recommend the perlref and perldsc perldoc pages - they'll help you wrap your mind around references.