my $key_file = 'key_file.txt';
my $encrypted = '...';
my $decrypted = '...';

my $my_key = `openssl enc -bf-cbc -d -in \Q$key_file\E -k encrypt`;
my $cipher = Crypt::CBC->new({ key => $my_key, cipher => 'Rijndael' });

open(my $FH_decrypted, '>', $decrypted)
   or die("Unable to create decrypted file \"$decrypted\": $!\n");
binmode $FH_decrypted;
flock $FH_decrypted, LOCK_EX;

open(my $FH_encrypted, '<', $encrypted)
   or die("Unable to open encrypted file \"$encrypted\": $!\n");
binmode $FH_encrypted;
flock $FH_encrypted, LOCK_SH;

while (!eof($FH_encrypted)) {
   print $FH_decrypted $cipher->decrypt(read_str($FH_encrypted));
}