Another way of doing it: 
$n=2;    # x-1
$chr="e";
$rep="1";
$a="Terence and Philip are sweet";
while ($a=~s/^((?:.*?$chr){$n})(.*?)$chr/$1$2$rep/) {}

[download]

--ZZamboni

Update: At ar0n's suggestion, the last line can also be written as: 

1 while ($a=~s/^((?:.*?$chr){$n})(.*?)$chr/$1$2$rep/);

[download]
which results in almost exactly what sachmet wrote. I like sachmet's answer better too because it uses only one capturing set of parenthesis.

In reply to Re: Replacing a given character starting with the xth occurence in a string by ZZamboni
in thread Replacing a given character starting with the xth occurence in a string by Anonymous Monk

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