As a corollary, if "abc" is placed before "\K", the substitution appends instead of replaces.

# 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;
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Of course, the "{1}" is optional in the last pattern.