Another approach. Identifies 'real' decimal octet IP addresses. Matches  'x' replacements to actual octet digits if that's really what you want to do.

c:\@Work\Perl\monks>perl -wMstrict -le
"use Regexp::Common qw(net);
 ;;
 for my $s (
   '1.2.3.4', '1.22.111.222',
   'a1.2.3.4a b1.22.111.221b',
   '999.9.9.9', '9.9.9.999',
   ) {
   printf qq{'$s' -> };
   (my $t = $s) =~
     s{ (?<! \d) ($RE{net}{IPv4}) (?! \d) }
      { (my $ip = $1) =~ s{ \d (?= \d* [.]) }{x}xmsg;  $ip; }xmsge;
   print qq{'$t'};
   }
"
'1.2.3.4' -> 'x.x.x.4'
'1.22.111.222' -> 'x.xx.xxx.222'
'a1.2.3.4a b1.22.111.221b' -> 'ax.x.x.4a bx.xx.xxx.221b'
'999.9.9.9' -> '999.9.9.9'
'9.9.9.999' -> '9.9.9.999'
[download]

Note that if you're using Perl 5.14+, the  /r substitution modifier makes the expression a bit simpler.

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.014;
 ;;
 use Regexp::Common qw(net);
 ;;
 for my $s (
   '1.2.3.4', '1.22.111.222',
   'a111.22.3.4a b1.22.111.221b',
   '999.9.9.9', '9.9.9.999',
   ) {
   printf qq{'$s' -> };
   my $t = $s =~ s{ (?<! \d) ($RE{net}{IPv4}) (?! \d)  }
                  { $1 =~ s{ \d (?= \d* [.]) }{x}xmsgr }xmsger;
   print qq{'$t'};
   }
"
'1.2.3.4' -> 'x.x.x.4'
'1.22.111.222' -> 'x.xx.xxx.222'
'a111.22.3.4a b1.22.111.221b' -> 'axxx.xx.x.4a bx.xx.xxx.221b'
'999.9.9.9' -> '999.9.9.9'
'9.9.9.999' -> '9.9.9.999'
[download]