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

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

In reply to Re: Doubley surprised by fat commas (updated) by LanX
in thread Doubley surprised by fat commas by GrandFather

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.