I can't seem to locate definitive sources at the moment, but it's mentioned often[1]. My general understanding of it is that the MOP is heavily influenced by CLOS, and the object system heavily influenced by the Smalltalk traits paper, which may itself have heavy CLOS influences. The easiest way to figure out just how similar they are may be to look at the Perl 6 docs on both the MOP[2] and Objects[3]. There's also the synopsis[4] documents which guide the design.

In addition, Perl 5's Moose[5] object system (and relatives Moo, Mouse, etc) are also heavily influenced by Perl 6, CLOS and Smalltalk.

1: http://irclog.perlgeek.de/perl6/search/?nick=&q=clos

2: http://doc.perl6.org/language/mop

3: http://doc.perl6.org/language/classtut

4: http://design.perl6.org/

5: http://moose.iinteractive.com/en/