You may try something like the below.

#!/bin/perl -w
use strict;

my %hash;
while (<DATA>) {
  $hash{$1}{$2}++ if ($_ =~ /.+success\s+(.+)\:(.+)/) ;
}

for my $user (keys %hash) {
  print $user."\n";
  print "==>".$_."(".$hash{$user}{$_}.")\n" for (keys %{$hash{$user}})
+;
  print $1." total:".scalar(keys %{$hash{$user}})." Unique IP's \n\n" 
+ if ($user =~ /(.+)\@/);
}


__DATA__
Jul 28 13:42:27 mail vpopmail[47985]: vchkpw-smtp: (PLAIN) login succe
+ss joe@example.com:192.168.250.251
Jul 28 13:42:28 mail vpopmail[47992]: vchkpw-smtp: (PLAIN) login succe
+ss joe@example.com:192.168.5.23
Jul 28 13:42:29 mail vpopmail[47994]: vchkpw-smtp: (PLAIN) login succe
+ss sally@example.com:192.168.10.28
Jul 28 13:42:27 mail vpopmail[47985]: vchkpw-smtp: (PLAIN) login succe
+ss fred@example.com:192.168.8.8
Jul 28 13:42:28 mail vpopmail[47992]: vchkpw-smtp: (PLAIN) login succe
+ss joe@example.com:192.168.5.23
Jul 28 13:42:29 mail vpopmail[47994]: vchkpw-smtp: (PLAIN) login succe
+ss harry@example.com:192.168.10.5
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