From perlfaq4: How do I change the Nth occurrence of something?

For example:

$ perl -le'
my $x = "ababababab";
print $x;
my $count = 0;
$x =~ s/a/ ++$count <= 3 ? "B" : "a" /eg;
print $x;
'
ababababab
BbBbBbabab
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