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

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
[download]

If your file is small, just read it into an array first:

open F, $file or die $!;
my @data = <F>;
for my $i ( 0..$#data ) {
    if ( $i == 0 or $i == $#data ) {
        #first or last line so do whatever
        print $data[$i];
    }
    else {
        # not first or last line
        $data[$i] =~ s#^(\d{16})#$_ = $1; tr/0-9/A-J/; $_#e
        print $data[$i];
    }
}
[download]

Brilliant! This works fine.

I am curious to know how this statement works:

$data[$i] =~ s#^(\d{16})#$_ = $1; tr/0-9/A-J/; $_#e

I understand some pieces of it:
-> the s is for substitution; ^d16 is for first 16digits;
-> tr does the conversion. # delimits;


Can you please let me know, how the whole piece fits in to do the substitution.

thanks

s/this/that/
s#this#that#
s#this# "th" . "at" #e   <-- the /e means (e)valuate the RHS before do
+ing substitution

# so the LHS of our RE is just this
^        begin string
(        begin capture into $1
\d{16}   16 digits
)        end capture into $1


# when we get a match of 16 digits at the begining of the string then 
+we have 2 events. 
# first $1 contains them and 
# second the RHS of the RE gets evaluated.

$_ = 1;      # set a var to $1 so we can modify it (can't do to $1 as 
+read only)
tr/0-9/A-J/; # transliterate contents of $_ aka $1
$_;          # perl will eval this with the net result that 
             # our 'that' result is $_ which duly gets used
             # to replace our original digits
[download]

/e is quite handy at times. The reason we need the naked $_ at the end of the RHS is because when perl evaluates a function (like the RHS) the return value is the last thing Perl evaluated. Without the $_ this would be the return value from tr which is not the transliterated string itself but rather the integer count of the number of transliterations. So we put the $_ there. You could put "$_ hello" or anything else you liked and that is what would get subbed in.

cheers

tachyon