my $marker = "<marker>\n";
my $endmarker = "</marker>\n";

my $marker_length = length($endmarker);
$marker_length++ if $^O =~ /MSWin/; # because of the \n -> CRLF conversion

sub append {
	my ($file, $data) = @_;

	if (!-s $file) {
		open my $FH, '>', $file or die qq{Can't open "$file" : $^E\n};
		print $FH $marker, $data, $endmarker;
		close $FH;
	} else {
		open my $FH, '+<', $file or die qq{Can't open "$file" : $^E\n};
		seek $FH, -$marker_length, 2;
		print $FH $data, $endmarker;
		close $FH;
	}
}

append('text.xml', "<foo>ahoj</foo>\n");
append('text.xml', "<foo>cau</foo>\n");
append('text.xml', "<foo>co</foo>\n");
append('text.xml', "<foo>delas?</foo>\n");