Perl command line replace for unicode

I am trying to replace every word (stored in a tmp file called _id) with a number using shell script. It works fine except for unicode words, for which a number is generated but replacement using Perl does not work. The bash code under question is as below:

x=0
for id in `cat _id`; do
    echo $x $id
    perl -p -i -e "s/\b$id\b/$x/g" x_graph.dot
    x=$(($x + 1))
done 

Can someone please point out to where the bug is?