use Data::Dumper;

my %coords = (
  x => { a => "Red" },
  y => { b => "Green" }
);

modifyHash(\%coords);

sub modifyHash
{
  my ($c,) = @_;
  $c->{x}{a} = "circle";
  $c->{y}{b} = "square";
}

print Dumper \%coords;