$ cat Fringe.pl
#!/usr/bin/perl
use strict;

my $tree1 = [ 'd', [ 'c', [ 'a', 'b', ], ], ];
my $tree2 = [ [ 'd', 'c' ], [ 'a', 'b' ] ];
#my $tree2 = [ [ [ 'd', 'c', ], 'a', ], 'b', ];

my $ti1 = get_tree_iterator($tree1);
my $ti2 = get_tree_iterator($tree2);

my $cnt_mismatches=0;
while (1) {
    my ($L, $R) = ($ti1->(), $ti2->());
    ++$cnt_mismatches unless ($R eq $L) or (!$L and !$R);
    print "L=$L, R=$R\n";
    last if !defined $L;
}
print $cnt_mismatches ? "$cnt_mismatches mismatches" : "TREES MATCH!", "\n";

sub get_tree_iterator {
    my @stack = (shift);
    return sub {
        unshift @stack, @{shift @stack} while @stack and ref $stack[0] eq 'ARRAY';
        return shift @stack;
    }
}
$ perl Fringe.pl
L=d, R=d
L=c, R=c
L=a, R=a
L=b, R=b
L=, R=
TREES MATCH!