my $string = "rue' version='4' changeset='6456424' lat='36.7256674' lon='-4.4539391' />";



sub modify_lat_lon {

	my $lat  = shift; 		# lat='1.11'
	my $lon  = shift;		# lon='2.22'
	my $fill = shift; 		# whatever is between the stuff above

	$lat =~ s/-?[\d.,]+/new_lat/;
	$lon =~ s/-?[\d.,]+/new_lon/;


	# recombine the parts to one string
	return $lat . $fill . $lon;
}

# one way (lat .. lon) or the other (lon .. lat)
$string =~ s/(lat='[^']+')(.*)(lon='[^']+')/modify_lat_lon($1,$3,$2)/e 
 ||
$string =~ s/(lon='[^']+')(.*)(lat='[^']+')/modify_lat_lon($3,$1,$2)/e;

print $string, $/;