... what does this line:

(my $mask = $s1) =~ tr{-\x00-,.-\xff}{\x00\xff};

It copies the string  $s1 to the new lexical  $mask while simultaneously translating the characters of  $s1 with the  tr/// function. If you have Perl version 5.14+, you can simplify this statement somewhat by using the  /r modifier:
    my $mask = $s1 =~ tr{-\x00-,.-\xff}{\x00\xff}r;
See  tr/// in the Quote-Like Operators section in perlop.

... which is the null character?

It is the  \0 character (or byte), hex 0x00, octal 000 (also decimal 0). (Update: Along with its cousin 0xff, it's very useful for creating bit-masks for strings.)