use strict;
use XML::Simple;

local $/;  # just so i can slurp __DATA__

my $old_xml = XMLin(<DATA>,forcearray=>1);
my $new_xml;

foreach my $heading (@{ $old_xml->{'heading'} }) {
	my $level = delete $heading->{'level'};
	$new_xml->{"heading$level"} = $heading;
}

print XMLout($new_xml);

__DATA__
<xml>
<heading level="2">Introduction to Arguments</heading>
<heading level="3">
<index primary-key="procedures" secondary-key="definition, rest arguments in"/>
<index primary-key="rest arguments" secondary-key="specifying, in procedure definition"/>
Specifying Rest Arguments in a Procedure Definition
</heading>
</xml>