Since noone explained the solution, I will:

Perl will absorb everything that starts with a dash until it meets something that doesn't start with dash or until it meets "--". The code attached to -e is considered part of -e, so it doesn't interrupt the scanning for options.

-s scans whatever is left would otherwise go to @ARGV.

     Interpreted as perl options
     vv vvvvvvvvvv vvvvvv 
perl -s -e "print" -foo=a a b
                          ^ ^
                          @ARGV, which -s scans

     Interpreted as perl options
     vv
perl -s script.pl -foo=a a b
                  ^^^^^^ ^ ^
                  @ARGV, which -s scans


     Interpreted as perl options
     vv vvvvvvvvvv vv
perl -s -e "print" -- -foo=a a b
                      ^^^^^^ ^ ^
                      @ARGV, which -s scans
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