You can use quotemeta, either as a function, or as \Q...\E (update: Relashed my slashes, thanks ikegami): 
$b = quotemeta $b;
$a =~ s/$b/$c/g;

#or

$a =~ s/\Q$b/$c/g; # \E not needed at end of pattern

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In reply to Re: s/$foo/$bar/ when $foo has metacharacters in it by FunkyMonk
in thread s/$foo/$bar/ when $foo has metacharacters in it by bcrowell2

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