Allow me to specify file format through XML, not coding.

You mean something like:

use XML::Simple;
use Parse::FixedLength;

my $x = XMLin(<<XMLEND);
<format>
<field>
  <fname>fld1</fname>
  <length>10</length>
</field>
<field>
  <fname>fld2</fname>
  <length>20</length>
</field>
<field>
  <fname>fld3</fname>
  <length>30</length>
</field>
</format>
XMLEND

my $fmt = [ map { "$_->{fname}:$_->{length}" } @{$x->{field}} ];

my $p = Parse::FixedLength->new($fmt);
[download]

Somehow I doubt you mean this, but you gotta start somewhere...(and I'm not clear on how XML makes this any better)