Perl has some super-low precedance operators (and, or and not). and and or can be used as statement modifiers. So, you can write:

open FILE, ">test.txt" or $failed_flag = 1;
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Or, if you add parentheses to your original statement, you can write:

open( FILE, ">test.txt" ) || $failed_flag = 1;
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That said, if you're using a modern perl, you are better off using the three-argument open and a lexical filehandle:

open my $FILE, ">", "test.txt" or $failed_flag = 1;
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See here to read more about precedance

update: missed a quote. Thanks toolic. not can't be used as a modifier.

In your open statement, || is evaluated before the comma, so your statement is parsed as open FILE, (">test.txt" || $failed_flag = 1); giving the syntax error.

Actually the  || operator also has higher precedence than the  = operator so it is parsed as open FILE, (">test.txt" || $failed_flag) = 1; which is why you get the error  Can't modify constant item in scalar assignment

You're absolutely right of course and I should have noticed that. Thanks for pointing it out.

You have a precedence problem.

General rule:
Use or to join statments together (where you'd otherwise use an if).
Use || when you'll use the result of the operation (i.e. in an assignment, to pass to a function, etc).

Omitting parens from function calls has a few pitfalls, including this one. You might be better off using them.

Try this:

open(FILE, ">test.txt") || $failed_flag = 1;
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It is often recommended these days to use the three-argument form of open with lexical filehandles, like this.

open my $testFH, '>', 'test.txt' or $failed_flag = 1;
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Cheers,

JohnGG

Update: I really must learn to type faster :(

Thanks all for the help and advice! For now I've switched from "||" to "or" but I'll definitely try to make a habit of using the new open statement as well, in this case it's not an option but it's good to know.
--------- brian

Why is it not an option?

unless(open FILE, ">test.txt"){
    #error handling code goes here
}
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