open returns true if it was successful:

if (open my $fo, '<', $ggg) {
    # Process the file.
} else {
   # Your code to exit.
}
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Update: Also note that parts of your code are repeated. That is against the DRY principle. Better:

{
    print "'$ggg' is missing:\nEnsure that this file is in the working
+ dir.\nWe are exiting.\n";
    my $answer;
    while ('q' ne lc $answer) {
        print "Please press 'q' and Enter to quit\n";
        chomp($answer = <STDIN>);
    }
    exit;
}
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