Imagine you have a huge XML. It has some header, then a <Foos> tag full of <Foo> tags and nothing more. And you have a tool that's able to somehow process such XML if only it either contained just one <Foo> or at least was not so big. Here is a tiny script that allows you to specify the name of the <Foo> tag, the huge file and floods your disk with little files containing one <Foo> each:

use strict;
use warnings;
no warnings 'uninitialized';

use XML::Rules;

die "Usage: $0 split_tag filename(s)\n" unless @ARGV >= 2;
my ($split_tag, @files) = @ARGV;

my $parser = XML::Rules->new(
    rules => [
        _default => 'raw',
        $split_tag => sub {
            my ($file, $id) = ( $_[4]->{parameters}{'file'}, ++$_[4]->
+{parameters}{'id'});
            $id = sprintf "%04d", $id;
            $file =~ s/(?:\.xml)?$/-$id.xml/i;

            if (ref $_[3]->[-1]{_content}) {
                $_[3]->[-1]{_content}[-1] =~ s/^.*(\n[^\n]+)$/$1/s;
            } else {
                $_[3]->[-1]{_content} =~ s/^.*(\n[^\n]+)$/$1/s;
            }

            print " $file\n";
            open my $FH, '>:utf8', $file or die qq{Can't create "$file
+": $^E\n};
            print $FH $_[4]->parentsToXML();
            print $FH $_[4]->ToXML( $_[0], $_[1]),"\n";
            print $FH $_[4]->closeParentsToXML();
            close $FH;

            return;
        }
    ]
);


foreach my $file (@files) {
    $parser->parsefile( $file, {file => $file});
}
[download]

That's it folks ;-)

And you only ever have the header plus one <Foo> in memory. Of course if you wanted to get rid of the <Foos>, rename the <Foo> or something, you can add that.