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.