The following works without using any explicit iterations. I haven't clocked its execution against the others, though.

$s = 'abc nbc cbs fox hbo sho cnn rox';
($orig, $num, $subst) = ('n', 2, '-');

$s =~ s/^(([^$orig]*$orig){$num})(.*)/$1/;
($t = $3) =~ s/$orig/$subst/g;
$s .= $t;

print $s
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This prints:

abc nbc cbs fox hbo sho cn- rox
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It works by cutting the string into two parts, applying the substitution to the second part (if there is one) and rejoining them.