The .* is greedy, as far as I can see that will always replace ithe last match if you have more than the number you need. 
my string = "foo_a_foo_b_foo_c_foo_d_foo_e_foo_f_foo"
my $pattern = "foo"
my $better = "bar"
my $n = 1;

$string =~ s/^(($pattern.*){$n})$pattern/$1$better/;

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would give: 
"foo_a_foo_b_foo_c_foo_d_foo_e_foo_f_bar"

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not 
"foo_a_bar_b_foo_c_foo_d_foo_e_foo_f_foo"

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Nuance

In reply to RE: RE: RE: Selective substitution: not in Perl? by nuance
in thread Selective substitution: not in Perl? by Anonymous Monk

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