here what B::Deparse sees, comments added

D:\tmp\pm>perl -MO=Deparse
print "a"=> =>"c";
print "d",,"e";
print ,"f";
print =>"d";
__END__
print 'a', 'c';
print 'd', 'e';
print($_), '???';      # comma seen as scalar comma separator 
'???', '???';          # "print" seen as hash-key, void constants fold
+ed away  
__DATA__


D:\tmp\pm>perl -MO=Deparse
print (, "b");
print (=> "g");
__END__
syntax error at - line 1, near "print (,"
syntax error at - line 2, near "print (=>"
- had compilation errors.
__DATA__
[download]

(I hope print as pseudo-method of file-handles was not too magic for this test.)

IIRC one is supposed to enter undef explicitly when wanted, like when ignoring elements in list assignments.

I think what's happening here is a form of flattening where leading commas at RHS are ignored.°

D:\tmp\pm>perl -MO=Deparse
print "a"=> => => "c";
__END__
print 'a', 'c';
__DATA__
[download]

update

°) yep already discussed here => gist: multiple "empty" commas are ignored