To successfully deal with strings in C code, you'll also need to deal with a few non-string constructs. In this case, the list is rather small since C strings are always delimited by double quotes ("), so you only need to deal with valid C constructs that might contain such a character.

Characters:   '"'
Comments:     /* " */
C++ comments: // "
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So you can write a cheap little parser to pull out quoted strings:

    my $parser= qr{
        \G                  # Don't skip anything
        (?:
          [^'"/]+           # Stuff we don't care about
        | '(?:[^\\']+|\\.)' # '"', '\'', '\\', 'param'
        | /\* .*? \*/       # A C comment
        | //[^\n]+          # A C++ comment
        | /                 # /, not a comment, division
        | "((?:[^\\"]+|\\.)*)"  # A quoted string ($1)
        | (.)               # An error ($2)
        )
    }xs;

    my $code= do { local($/); <CCODE> };
    my @strings;
    while(  $code =~ m/$parser/g  ) {
        if(  defined $1  ) {
            push @strings, $1;
        } elsif(  defined $2  ) {
            my $char= $2;
            my $pos= pos($code)-5;
            $pos= 0   if  $pos < 0;
            my $context= substr( $code, $pos, 10 );
            warn "Ignoring unexpected character ($char) in ($context)"
+;
        }
    }
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Update: Enlil was kind enough to point out that '[^']+' won't match '\''. I replaced that part. Note that I support the rather strange:

#define ctrl(char) ( 'char' & 31 )
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And here is a hint at how to extend it to support replacing strings:

#!/usr/bin/perl -p0777 -i.org

my $parser;
BEGIN {
    $parser= qr{
        \G                  # Don't skip anything
        (
          [^'"/]+           # Stuff we don't care about
        | '(?:[^\\']+|\\.)' # '"', '\'', '\\', 'param'
        | /\* .*? \*/       # A C comment
        | //[^\n]+          # A C++ comment
        | /                 # /, not a comment, division
        | "((?:[^\\"]+|\\.)*)"  # A quoted string ($2)
        | (.)               # An error ($3)
        )                   # Entire match ($1)
    }xs;
}

    s{$parser}{
        if(  defined $3  ) {
            my $char= $2;
            my $pos= pos($code)-5;
            $pos= 0   if  $pos < 0;
            my $context= substr( $code, $pos, 10 );
            warn "Ignoring unexpected character ($char) in ($context)"
+;
        }
        if(  defined $2  ) {
            my $string= $2;
            #... manipulate $string ...
            $string;
        } else {
            $1;
        }
    }g;
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To see test script,

/s