Is it necessary to redirect file to the script instead of letting <> read the argument?

Yep, otherwise it'll just hang waiting on STDIN.

Here's a solution that adheres to the rules but is awful sneaky

shell> cat > obfuwc.pl
#!/usr/bin/perl -lp -s
++(${$::{$-}}=()=<>)
^D
shell> chmod +x obfuwc.pl
shell> ./obfuwc.pl --=_ < file
[download]

So the actual code passes the /\w/ but sneaks through the rules by passing in _. I can't say I'm proud ;)
HTH

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broquaint