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

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

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

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