First, your call to ->max_depth doesn't do what you think it does. I think you should call it as:
JSON::XS->new->utf8->max_depth(100);
that is, leave the square brackets out.
Second, maybe your data structure cannot be serialized as JSON because it is circular? Have you inspected your data structure using Data::Dumper to see what it looks like?
#!perl -w
use strict;
use JSON::XS;
# Create a simple circular data structure:
my $output;
$output = ['foo'];
$output->[1] = $output;
my $json = JSON::XS->new->utf8->max_depth(100);
use Data::Dumper;
print Dumper $output;
print $json->encode($output);
__END__
json text or perl structure exceeds maximum nesting level (max_depth s
+et too low?) at tmp.pl line 13.
