Glad you found your problem, but a note of caution. If your ever going to upgrade from 5.005 to a newer version of Perl, and if there is any chance that any of your data will contain utf-8 characters, substr might cause you problems in that it deals in characters not bytes.

You might consider using unpack 'a100' or use bytes to avoid problems as and when you upgrade. A snippet to demonstrate the potential problem and both solutions.

#! perl -slw
use strict;
use 5.008;

my $utfdata = join'',map{ chr(0xffff + $_) } 1 .. 100;
print length $utfdata;                      #! Gives 100

my @bytes = unpack 'C*', $utfdata;
print scalar @bytes;                     #! Gives 400

my $chunk = substr $utfdata, 0, 100;
print length $chunk;                     #! Gives 100

@bytes = unpack 'C*', $chunk;
print scalar @bytes;                     #! Gives 400

{
    use bytes;
    print length $utfdata;               #! Gives 400
    print length $chunk;                 #! Gives 400
}

my @chunks = unpack '(a100)*', $utfdata;
print scalar @chunks;                    #! Gives 4
print "@{[ map{length} @chunks ]}";      #! Gives 100 100 100 100
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