Why the file-test operator does not work (Perl)?

In "Programming Perl" -w file-test operator described as:

–w File is writable by effective UID/GID.

I have two files:

-rwsrwxrwx 1 testuser testuser 226 Jul 20 20:31 script.pl
-rw-rw-r-- 1 testuser testuser  34 Jul 14 17:24 file.txt

suid is set on script.pl, so when I run it as user caligula, effective UID/GID should be the testuser's one. script.pl is:

#!/usr/bin/perl
use v5.14;
if (-w 'file.txt') {
    say "true";
}
else {
    say "false";
}

But when I run it caligula@ubuntu-host:~$ ./script.pl the output is always false. Why does that happen, may be I didn't understand the correct usage of that operator?

My appreciation.