# Open the file
open(FILE, "<file") || die;

# Run gpg
# Notice that we are not going to bother to try to
# make the file become stdin to gpg. We're just going
# to tell gpg which file descriptor it's already
# accessible as.
system("gpg", "--passphrase-fd=" . fileno(FILE),
    "--decrypt", "--output", "cleartext-filename",
    "ciphertext-filename");

close FILE;