my $input_filename = 'foo.xml';
my $temp_filename = 'bar.xml';
open my $ofh, '>', $temp_filename or die $!;
open my $ifh, '<', $input_filename or die $!;
while (<$ifh>) {
	s/foo/bar/g;
	print $ofh $_;
}
close $ifh;
close $ofh;

# now open $temp_filename with your XML parser