my %emoticons = (
  smiley => ':)',
  wink => ';)',
  );

# example:  <img alt="smiley">  becomes  :)
foreach my $e (keys %emoticons)
{
   $body =~ s{
               \< img
               [^>]*?
               alt = "$e"
               [^>]*?
               \>
             }
             {$emoticons{$e}}igex;
}