The problem is here:

while (<$in>)
        {            
        s/\band\b/$word/ig; # <- Here you make all the substitutions
                
                while (/\band\b/ig) # <-... and here nothing happens!
                    {
                    ++$count;
                    }
                    
        print($out "$_\n");
}
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s/// will return the number of occurrences, so...

while (<$in>)
        {            
        $code = s/\band\b/$word/ig; # <- Here you make all the substit
+utions
## UPDATE:
## $code += s/\band\b/$word/ig; # <- Here you make all
## Sorry for the mistake (thanks blazar!)
                    
        print($out "$_\n");
}
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should works

citromatik