This isn't pure regex, but it is the way I would have solved the problem. If matches $char, assigns the prematch to $match, and when the counter == $n it exits and prints $match. Kinda ugly, but works well!

$n = 5;
$char = 'l';
$string = 'hello my name is william gobbel-dy-gook liam';

$count = 0;
while ($string =~ /[$char]/g) {
    $match = $`;
    $count++;
    last if $count == $n;
}
print $match;
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