You want to use a negative look-ahead assertion (as described in 'perlre'): 
$string = "123456123789";
$a = 123;
$b = "abc";
$c = 456;
$string =~ s/$a(?!=$c)/$b/;

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That sets string to "123456abc789". (?=...) is a positive look-ahead, (?!=...) is a negative look-ahead. (?<=...) is a positive look-behind, (?<!...) is a negative look-behind.

In reply to Re: Substitute $a with $b only when a is not followed by $c by japhy
in thread Substitute $a with $b only when a is not followed by $c by Anonymous Monk

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