I thought there might be a way of reusing the existing code in Crypt::CBC, so I took a peek. Not only can code from Crypt::CBC be reused, Crypt::CBC already does what you want! The following encrypts:

use Crypt::CBC;

my $cipher = Crypt::CBC->new(...);

open(INFILE, '<', 'BIG_FILE.txt')
   or die("Unable to open input file: $!");

open(OUTFILE, '>', 'BIG_FILE.enc')
   or die("Unable to create output file: $!");

$cipher->start('encrypting');
while (read(INFILE, $buffer, 65536)) {
   print OUTFILE $cipher->crypt($buffer);
}
print OUTFILE $cipher->finish;
[download]

The following decrypts:

use Crypt::CBC;

my $cipher = Crypt::CBC->new(...);

open(INFILE, '<', 'BIG_FILE.enc')
   or die("Unable to open input file: $!");

open(OUTFILE, '>', 'BIG_FILE.txt')
   or die("Unable to create output file: $!");

$cipher->start('decrypting');
while (read(INFILE, $buffer, 65536)) {
   print OUTFILE $cipher->crypt($buffer);
}
print OUTFILE $cipher->finish;
[download]