If ``/'' is the delimiter then the initial m is optional. With the m you can use any pair of non-alphanumeric, non-whitespace characters as delimiters.

C:\>type test.pl
$re =~ "^0\$";
$re =~ m"^0\$";
$re =~ '^0\$';
$re =~ m'^0\$';
$re =~ <^0\$>;
$re =~ m<^0\$>;

C:\>perl -MO=Deparse test.pl
$re =~ /^0$/;
$re =~ /^0\$/;
$re =~ /^0\$/;
$re =~ /^0\$/;
$re =~ /CORE::GLOBAL::glob('^0$', 0)/;
$re =~ /^0\$/;
test.pl syntax OK
C:\>