Maybe something like the following? It is quite simple and avoids the difficulty of translating pointers between C and Perl.

#!/usr/bin/perl
use strict;
use warnings;
use Inline 'C';


foreach (1..15) {
    my $tree = new_tree();
    die "failed to allocate tree" if($tree < 0);

    print "node count of tree $tree is: " . node_count($tree), "\n";
}

__END__

__C__

#define NMAX 10

struct tree {
    int    size;
} trees[NMAX];

int td = 0;

int new_tree() {
    if(td < NMAX) {
        trees[td].size = td * 2;
        return(td++);
    } else {
        return(-1);
    }
}

int node_count(int td) {
    return(trees[td].size);
}
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You could allocate memory dynamically if that were appropriate, perhaps simplifying the trees structure to an array of pointers to allocated memory. With a little more sophistication, you could free tree descriptors, keep track of which ones were in use, etc.