There's no difference, since an allocated hash is already set up for being blessed. 

push @l,{} for 1..3e7;     # allocate 3 mill. anon hashes
system "ps -o vsz= -p $$"; # get memory usage of process
$_ = bless $_ for @l;      # bless each hash into 'main'
system "ps -o vsz= -p $$"; # again get memory usage
__END__
2650968
2650968

[download]

which shows: making an anonymous hash into an object means blessing it into a namespace. That operation signifies no overhead as far as memory is concerned, since the namespace bits are already allocated in the first place.

update: added comments

update: AnomalousMonk correctly noted that 3e7 for 3 millions is wrong by an order of magnitude. Who else noticed? ;)

perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'

In reply to Re: Perl object memory overhead by shmem
in thread Perl object memory overhead by Ineffectual

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