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# Appends to the 3rd occurence of 'abc' from start-of-line
($a = $ref) =~ s/^(?:.*?abc\K){3}/___/;
...
# omit ^ to append to every 1st occurence of 'abc'
($a = $ref) =~ s/(?:.*?abc\K){1}/___/g;
print $a;