Now the Simple way :)

#! perl -slw
use strict;
use XML::Simple;

my $xml = XMLin( \*DATA, KeepRoot => 1 );

$xml->{policy}{Rule} = $xml->{policy}{policyrules}{Rule};
delete $xml->{policy}{policyrules};

print XMLout( $xml, KeepRoot => 1 );

__END__
<?xml version="1.0" encoding="UTF-8"?>
<policy name="Cur_policy" version="3.2.1">
    <policyrules>
        <Rule Action="allow" Enabled="true" RuleID="F68E32">
            <source addr="10.4.5.3" srcport="any"/>
            <dest addr="199.5.4.3" destport="80"/>
        </Rule>
        <Rule Action="allow" Enabled="true" RuleID="78E21D">
            <source addr="10.4.0.0-10.4.255.255" srcport="any"/>
            <dest addr="any" destport="80"/>
        </Rule>
    </policyrules>
</policy>
[download]

Produces:

C:\test>xmljunk.pl
<policy name="Cur_policy" version="3.2.1">
  <Rule Action="allow" Enabled="true" RuleID="F68E32">
    <dest addr="199.5.4.3" destport="80" />
    <source addr="10.4.5.3" srcport="any" />
  </Rule>
  <Rule Action="allow" Enabled="true" RuleID="78E21D">
    <dest addr="any" destport="80" />
    <source addr="10.4.0.0-10.4.255.255" srcport="any" />
  </Rule>
</policy>
[download]