For cheap thrills, a Perl 5 version using reduce:

use List::Util qw(reduce);
my $s = "ZBBBCZZ";
my $x = reduce { if (@$a && index($a->[-1], $b) >= 0) { $a->[-1] .= $b
+ } else { push @$a, $b } $a } [], split //, $s;
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