just change the "1, 8, 1" to "188, 8, 16" to match your case... those values are just for testing

#!perl -w

# cases above the blank line should be replaced
# successfully, below should not

use strict;

my ($len1, $len2, $len3) = (1, 8, 1);
my $valid = qr/^.{$len1}(\d{$len2}).{$len3}$/;

while (<DATA>)
{
        next unless $_ =~ $valid;
        next unless $1 ne "00000000";
        substr($_, $len1, $len2) = "20020101";
        print;
}

__DATA__
X12345678X
X14322678X
X12534654X
X12345678X

X00000000X
12345678X
X12345678
XX
12345678
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