Says cLive ;-):

$::^M is equivalent to $main::^M - 'main' is the default namespace.

Actually they're both syntax errors.

If they weren't syntax errors, then yes, the variable $::^M is identical to $main::^M. But not because main is the default namespace. It's because the empty stash name is interpreted as though you had said main instead. See line 579 of gv.c:

            if (!*name)
                return gv ? gv : (GV*)*hv_fetch(PL_defstash, "main::",
+ 6, TRUE);
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If the name turns out to have been empty, Perl looks up the main stash instead.

--
Mark Dominus
Perl Paraphernalia