It depends on how your header and trailer data look like. If they do not start with numerics, then it's easy...

while (<$filehandle>) {
    tr/blah/blah/ if /^\d{16}/; # do the translation
    print;                      # output result
}
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If your header and trailer look similar to the rest of the data, and if your file is small, you could read everything in to memory first...

my @data = <$filehandle>;
print $data[0];     # print first line
for (1..$#data-1) { # skip first and last line
  $data[$_] =~ tr/blah/blah;
  print $data[$_];
}
print $data[-1];    # print last line
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Otherwise it will get a bit messy.