TIMTOADY:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $ssn = '# SSN=123-12-1234';
 print qq{'$ssn'};
 ;;
 $ssn =~ s{ (?! \d{1,4} \z) \d }{X}xmsg;
 print qq{'$ssn'};
"
'# SSN=123-12-1234'
'# SSN=XXX-XX-1234'
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