# only Perl 5.10+
my $a;
my $ref = "abc x abc y abc z abc\n";

# search 3rd occurence of 'abc' from start-of-line
($a = $ref) =~ s/^(?:.*?\Kabc){3}/___/;
print $a;

# search 4th occurence of 'abc' from start-of-line
($a = $ref) =~ s/^(?:.*?\Kabc){4}/___/;
print $a;

# omit ^ to search every 2nd occurence of 'abc'
($a = $ref) =~ s/(?:.*?\Kabc){2}/___/g;
print $a;

# omit ^ to search every 1st occurence of 'abc'
($a = $ref) =~ s/(?:.*?\Kabc){1}/___/g;
print $a;

__END__

abc x abc y ___ z abc
abc x abc y abc z ___
abc x ___ y abc z ___
___ x ___ y ___ z ___