[kw-pm] Algorithm

[Robert Pike]

Can anyone suggest the easiest / fastest way (with fewest lines of code) to take the xml below and create a hash with only the ids in the hash while preserving the hierarchy and using XML::Simple?
 
<!DOCTYPE TREELAYOUT [  
<!ELEMENT treeMap (treeTop*)> 
<!ELEMENT treeTop (blockInfo, treeTop*)> 
<!ELEMENT blockInfo (id, name, description, field1)> 
<!ELEMENT id (#PCDATA)> 
<!ELEMENT name (#PCDATA)> 
<!ELEMENT description (#PCDATA)> 
<!ELEMENT field1 (#PCDATA)> 
]>
 
 
<?xml version="1.0" encoding="iso-8859-1"?>
<treeMap>
    <treeTop>
     <blockInfo>
      <id>111</id>
   <name>Item 111</name>
   <description>Item Description</description>
   <field1>Additional Info</field1>
  </blockInfo>
  
  <treeTop>
      <blockInfo>
          <id>111b</id>
    <name>Item 111b</name>
       <description>Item Description</description>
       <field1>Additional Info</field1>
      </blockInfo>
  </treeTop>
  <treeTop>
      <blockInfo>
          <id>222b</id>
    <name>Item 222b</name>
       <description>Item Description</description>
       <field1>Additional Info</field1>
      </blockInfo>
   <treeTop>
       <blockInfo>
              <id>111c</id>
        <name>Item 111c</name>
           <description>Item Description</description>
           <field1>Additional Info</field1>
          </blockInfo>
   </treeTop>
   <treeTop>
       <blockInfo>
              <id>222c</id>
        <name>Item 222c</name>
           <description>Item Description</description>
           <field1>Additional Info</field1>
          </blockInfo>
   </treeTop>
  </treeTop>
  <treeTop>
   <blockInfo>
    <id>333b</id>
    <name>Item 333b</name>
    <description>Item Description</description>
    <field1>Additional Info</field1>
   </blockInfo>
  </treeTop>
 </treeTop>
 <treeTop>
     <blockInfo>
   <id>222</id>
   <name>Item 222</name>
   <description>Item Description</description>
   <field1>Additional Info</field1>
  </blockInfo>
 </treeTop>
</treeMap>
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