# assume that the long string from XML::Simple is assigned to $_:

s/></>\n</g;  # put line breaks between all adjacent tags
my @lines = split( /\n/ );
my $indent = 0;
$_ = "";
foreach my $l ( @lines ) {
    $indent -=2 if ( $l =~ /<\// );  # close tag decreases indent
    $_ .= " " x $indent . "$l\n";
    $indent +=2 if ( $l =~ /^<\w/ ); # open tag increases indent
}
print;