You can use %ENV to access environment variables, or as a trick you can use the -s switch:

$ cat test.txt 
one
two
three
$ part="C:00000092666270:53882159774"
$ echo $part
C:00000092666270:53882159774
$ perl -pse 'print qq{foo = [ "$bar" ]\n} if $. == 3' -- -bar="$part" 
+test.txt
one
two
foo = [ "C:00000092666270:53882159774" ]
three
$ export part
$ perl -pe 'print qq{foo = [ "$ENV{part}" ]\n} if $. == 3' test.txt
one
two
foo = [ "C:00000092666270:53882159774" ]
three
[download]

I removed the -i for demonstration purposes, you can just add it back in, also I used -p instead of -n to save a bit of typing, but neither of those changes are necessary for the ideas I'm showing to work.

Although personally, I usually prefer to just write the whole thing in Perl instead of intermixing Perl and shell scripting.