Needless sprintf, needless match, and you should probably separate the number from the stars with a space. I'd also require at least 3 stars to trigger a match. 

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

[download]

Optimised: (requires 5.10+) 

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

[download]

As a one-liner: (Second version requires 5.10+) 

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

[download]

In reply to Re^2: Numbering instances of a String in a file by ikegami
in thread Numbering instances of a String in a file by Anonymous Monk

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