my %hashA = ( orange=>'good',
              apple=>'bad' );

my %hashB = ( here=>'today',
              gone=>'tomorrow' );

myfunction(\%hashA,\%hashB); # send references to the hashes

sub myfunction
{
  my $hashrefA = shift; # (or $_[0])
  my $hashrefB = shift; # (or $_[1])

  my %hashA = %$hashrefA; # treat the refs as hashes and
  my %hashB = %$hashrefB; # copy the contents

  $hashA{extravalue} = "change seen in the subroutine only";
  $hashrefA->{extravalue} = "change seen in the main program";

}