First of all, you probably want /^.{188}00000000.{16}$/ so that you match from the beginning to the end of the string... what you have could match a longer line that matched those criteria, if there was one. If the 8 digit number will never start with a zero (assuming it is already a date), then

perl -lne 'print if s/^(.{188})[1-9]\d{7}(.{16})$/${1}20020101$2/' ori
+ginal.txt > new.txt
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would work. Theoretically you could use lookahead and such to negate the need of capturing parens (Upd: As ishnid shows).

There is probably a way to do it in one regexp, but I can't think of it now, so if zero at the start is a possibility.

while(<>) {
 next unless /^(.{188})(\d{8})(.{16})$/;
 print $1,"20020101$3\n" unless $2 eq '00000000';
}
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untested, but should work.