#! perl -slw
use strict;
use constant INDENT_SIZE => 4;

my @xml = (
'<new>',
'<mp3>',
'<song title>',
'</song title>',
'</mp3>',
'</new>',
);

my $indent = 0;

for (@xml) {
	$indent -= INDENT_SIZE if m|^</|;
	print ' ' x $indent, $_;
	$indent += INDENT_SIZE if m|^<[^/]|;
}
__END__
C:\test>215754
<new>
    <mp3>
        <song title>
        </song title>
    </mp3>
</new>

C:\test>