my $cnt = '0000';
while (<>) {
    s/^(\*{3,})$/"$1 ". ++$cnt/e;
    print;
}

##</code><code>##

my $cnt = '0000';
while (<>) {
    s/^\*{3,}\K$/" ". ++$cnt/e;
    print;
}

##</code><code>##

perl -pe'BEGIN { $cnt = '0000' }  s/^(\*{3,})$/"$1 ". ++$cnt/e'
perl -pe'BEGIN { $cnt = '0000' }  s/^\*{3,}\K$/" ". ++$cnt/e'