First if you're not already using it, I recommend Use strict and warnings, as your code currently has a few uninitialized variables of questionable origin. Next, that modules seems klunky for this purpose, I'd use XML::Rules (I don't print out the same thing as you, since I don't know where you want to get some of what you want):

use strict;
use warnings;

use XML::Rules;

my $xml = <<XML;
<FIXML r="20030618" s="20040109" v="4.4" xr="FIA" xv="1">
<Batch>
   <MktDataFull RptID="13793742" BizDt="2011-12-23">
        <Instrmt Sym="MID" MMY="20120317"/>
             <Full Typ="5" Px="5.303128"/>
             <Full Typ="D" Px="884.91"/>
   </MktDataFull>
    <MktDataFull RptID="14536119" BizDt="2011-12-23">
        <Instrmt Sym="MID" MMY="20120218"/>
            <Full Typ="5" Px="214.007661"/>
            <Full Typ="D" Px="884.91"/>
    </MktDataFull>
</Batch>
</FIXML>
XML

my @rules = (
  MktDataFull => sub {
    my $data = $_[1];
    for my $full (@{$data->{Full}}) {
      print join(",", @$data{qw(RptID BizDt Sym MMY)}, @$full{qw(Typ P
+x)}), "\n";
    }
    return;
  },
  Instrmt => 'pass',
  Full => 'as array',
);

my $xr = XML::Rules->new(rules => \@rules, stripspaces => 3);
$xr->parse($xml);
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