This is a precedence issue

shell> perl -MO=Deparse,-p -

defined(%stores) ? ( $wml = join "<br/>\n",map { "<a href='$params{URL
+}/op?optin&amp;store=$_'>$stores{$_}->{'store_t_address'}</a>" }(keys
+ %stores) ):  $wml = "Pas de résultat<br/><br/>" ;

^D
Assignment to both a list and a scalar at - line 2, near ""Pas de résu
+ltat<br/><br/>" ;"
- had compilation errors.

((defined(%stores) ?
    ($wml = join("<br/>\n",
            map({"<a href='$params{'URL'}/op?optin&amp;store=$_'>$stor
+es{$_}{'store_t_address'}</a>";} 
            keys(%stores))))
  :
     $wml) = "Pas de r\351sultat<br/><br/>");
[download]

As you can see in the B::Deparsed code (with the output slightly formatted) the assignment is occurring outside the ?: statement. Just put parentheses around the last statement and it should work fine.
HTH

_________
broquaint