Plan B was going to be to go with an XML config file...

You mean something like this?

$ cat test.xml
<?xml version="1.0"?>
<filelist>
<file name="file1">
  <field name="field1" start="50"  length="20"/>
  <field name="field2" start="80"  length="20"/>
  <field name="field3" start="100"  length="20"/>
</file>
<file name="file2">
  <field name="field1" start="52"  length="22"/>
  <field name="field2" start="82"  length="22"/>
  <field name="field3" start="120"  length="22"/>
</file>
<file name="file3">
  <field name="field1" start="53"  length="23"/>
  <field name="field2" start="83"  length="23"/>
  <field name="field3" start="130"  length="23"/>
</file>
</filelist>


$ perl -MXML::Simple -MData::Dumper -e '$x=XMLin("test.xml"); print Du
+mper($x)'

$VAR1 = {
          'file' => {
                    'file2' => {
                               'field' => {
                                          'field1' => {
                                                      'length' => '22'
+,
                                                      'start' => '52'
                                                    },
                                          'field2' => {
                                                      'length' => '22'
+,
                                                      'start' => '82'
                                                    },
                                          'field3' => {
                                                      'length' => '22'
+,
                                                      'start' => '120'
                                                    }
                                        }
                             },
                    'file1' => {
                               'field' => {
                                          'field1' => {
                                                      'length' => '20'
+,
                                                      'start' => '50'
                                                    },
                                          'field2' => {
                                                      'length' => '20'
+,
                                                      'start' => '80'
                                                    },
                                          'field3' => {
                                                      'length' => '20'
+,
                                                      'start' => '100'
                                                    }
                                        }
                             },
                    'file3' => {
                               'field' => {
                                          'field1' => {
                                                      'length' => '23'
+,
                                                      'start' => '53'
                                                    },
                                          'field2' => {
                                                      'length' => '23'
+,
                                                      'start' => '83'
                                                    },
                                          'field3' => {
                                                      'length' => '23'
+,
                                                      'start' => '130'
                                                    }
                                        }
                             }
                  }
        };
[download]

But... what? Looks pretty simple to me. I'd rather not have all those data values hard-coded in the perl script itself, and reading them from a basic xml file seems like a no-brainer.

I guess it's a little goofy that there are two layers in XML::Simple's "default" hash structure that are sort of useless ("file" and "field"), but if you tried a different way of structuring the xml (and/or spent some time with the XML::Simple man page), you could probably improve on that. (Or you could just use the default as-is and get done quicker.)