Good question, I suspect that it is an optimization. Trying 

my $str = "bea" x 100;
my $re  = qr/(?:be|ea|a)/;

sub atomic {
    use re 'debug';
    say 'Matched $re+\d' if $str =~ m/$re+\d/;
}

sub possessive {
    use re 'debug';
    say 'Matched $re++\d' if $str =~ m/$re++\d/;
}
atomic();
possessive();

[download]
The regexes compile to 
# atomic
Compiling REx "(?-xism:(?:be|ea|a))+\d"
Final program:
   1: CURLYX[0] {1,32767} (14)
   3:   TRIE-EXACT[abe] (13)
        <be> 
        <ea> 
        <a> 
  13: WHILEM[1/1] (0)
  14: NOTHING (15)
  15: DIGIT (16)
  16: END (0)
minlen 2 

#possessive
Compiling REx "(?-xism:(?:be|ea|a))++\d"
Final program:
   1: SUSPEND (19)
   3:   CURLYX[0] {1,32767} (16)
   5:     TRIE-EXACT[abe] (15)
          <be> 
          <ea> 
          <a> 
  15:   WHILEM[1/1] (0)
  16:   NOTHING (17)
  17:   SUCCEED (0)
  18: TAIL (19)
  19: DIGIT (20)
  20: END (0)
minlen 2 

# regex.ato and regex.pos contain the output including
# the Compiling REx message
michael@lnx-main:working> wc regex.ato regex.pos
   10650    70471  1837158 regex.ato
  229368  1989603 47988305 regex.pos

[download]
As you can see it is doing a lot more. I don't know why but it will be interesting to find out;)

Update 1: It may be something to do with caching. Looking in the re debug output I find 

michael@lnx-main:working> grep Detected regex.ato
whilem: Detected a super-linear match, switching on caching...
michael@lnx-main:working> grep Detected regex.pos
whilem: Detected a super-linear match, switching on caching...
# Now look for this line
# whilem: (cache) already tried at this position...
michael@lnx-main:working> grep '(cache)' regex.ato | wc
    298    2086   26426
michael@lnx-main:working> grep '(cache)' regex.pos | wc
      0       0       0

[download]
At this point I am prepared to confess that I will struggling a bit to dig in deeper;-)

Update 2: The SUSPEND/TAIL pair look like where any interested monk should start. In perldebguts I found this 

# Do nothing
NOTHING    no    Match empty string.
# A variant of above which delimits a group, thus stops optimizations
TAIL    no    Match empty string. Can jump here from outside.

[download]

In reply to Re^3: Performance of possessive quantifiers by hipowls
in thread Performance of possessive quantifiers by moritz

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.