while (<$filehandle>) {
    tr/blah/blah/ if /^\d{16}/; # do the translation
    print;                      # output result
}

##</code><code>##

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