use strict;
use warnings;
use XML::Rules;

my $rules =XML::Rules->new(
	style => 'parser',
	rules => {
		'inline-formula'  => sub {
			# (tag, attrs, context, parents, parser)
			open my $OUT, '>:utf8', $_[1]{id} . ".xml"; # open file named after the contents of the id attribute
			print $OUT $_[4]->toXML($_[0], $_[1]); # serialize the tag with attributes
			close $OUT;
		},
		'_default' => 'raw', # make sure we keep everything else intact
	}
);

$rules->parse( *DATA );

__DATA__
<data>
<inline-formula id="ieqn-1"><alternatives><mml:math display="inline"><mml:mi>&#964;</mml:mi></mml:math></alternatives></inline-formula>

<inline-formula id="ieqn-2"><alternatives><mml:math display="inline"><mml:mi>&#964;</mml:mi></mml:math></alternatives></inline-formula>

<inline-formula id="ieqn-3"><alternatives><mml:math display="inline"><mml:mi>&#964;</mml:mi></mml:math></alternatives></inline-formula>
</data>