You can simplify your code a lot by making $res a package variable. 
my @result =
   map
   {
       local our $res;
       m
       {(?x)
          ^
          (?(?=([a-z])$)
             (?{ $res = qq{$1=>fn($1)}; })
             |
             (?(?=([a-z])=([a-z])$)
                (?{ $res = qq{$2=>$3}; })
                |
                (?(?=([a-z])=\#([a-z])$)
                   (?{ $res = qq{$4=>fn($5)}; })
                   |
                   (?{ $res = q{???}; })
                )
             )
          )
       };
       $res
   }
   split m{,}, $str;

[download]

Of course, the above is just an obfuscated way of doing 

my @result =
   map
   {
       (/([a-z])$/
          ? qq{$1=>fn($2)}
          : (/([a-z])=([a-z])$/
               ? qq{$1=>$2}
               : (/([a-z])=\#([a-z])$/
                    ? qq{$1=>fn($2)}
                    : q{???}
                 )
            )
       )
   }
   split m{,}, $str;

[download]

By no means do I consider the second snippet acceptable either.

By the way, why did you introduce limits on what the values can be? Nowhere did the OP imply the values would be single lowercase letters. If you remove that arbitrary limit, you can get rid of the "???" case.

Why did you also changed the output, returning a string where a pair of values should have been returned?

In reply to Re^2: map a list by ikegami
in thread map a list by evil_otto

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.