use XML::Simple;

$data  = XMLin( \*DATA, KeepRoot => 1 );
@attrs = map { keys %{ $_->{attributes}{attribute} } }
         values %{ $data->{dataschemas}{dataschema} };

print "@attrs";

__DATA__
<dataschemas>
  <dataschema name="varyingName1">
   <attributes>
    <attribute>
     <one>1</one>
     <two>2</two>
    </attribute>
   </attributes>
  </dataschema>
  <dataschema name="varyingName2">
   <attributes>
    <attribute>
      <three>3</three>
      <four>4</four>
    </attribute>
   </attributes>
  </dataschema>
</dataschemas>