A recent discussion found this to be the fastest way, and it's pretty slick:

$_='abcabbcabbbbcabcabbcab';
my $n = 3;         ## Find the $nth occurrence
my $pat = qr/ab+/; ## of this pattern
my ($NthMatch) = /(?:.*?($pat)){$n}/;
print "Match #$n looks like $NthMatch\n";
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The regex is: find the pattern (optionally preceded by something that isn't the pattern) n times. The pattern is in parens, so the final match will be returned.