In addition to choroba's comments, I would say that this code line:

if(isdigit (\w)
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has several syntax mistakes. First, there is no closing parenthesis for the if conditional. Then, there is no opening curly brace for the code to execute if the conditional returns a true value. And finally, the (\w) argument doesn't make much sense outside of a regex.

But you probably don't need this conditional anyway, since your substitution on the next line can be modified to match only numbers, with something like this (untested):

s/(\d+)/num2en($1)/ge;
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