I happened to notice an SoPW node where the OP's code passed $ARGV[0] to "open()" without checking if there was any value there, and I was intrigued about what Perl does with this sort of potential error. When I tried it out, I got a surprise: 
#!/usr/bin/perl -w

## test 1: open for input

open( I, $ARGV[0] ) or die $!;

while(<I>) {print}

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If I run that with no args (@ARGV empty), it sends a warning ("use of uninitialized value in open") and dies ("no such file or directory"). But then: 
#!/usr/bin/perl -w

## test 2: open for output

open( O, '>', $ARGV[0] ) or die $!;

print O "this is a test\n";

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If I run that with no args, I get no warning, no die, and no output file -- no output of any sort, anywhere.

Now, if I rephrase that second open statement to the 2-arg version --  open( O, ">$ARGV[0]" ) or die $!; -- then I get the expected warning and dying messages.

For that matter, if I convert the first test to use a three-arg open --  open( I, '<', $ARGV[0] ) -- I don't get any warning or die (and "use warnings" doesn't help, either).

I find this a bit disturbing (same behavior on macosx 5.8.1 and freebsd 5.8.6). Am I missing something?

update: special thanks to Transient for an illuminating tour of the relevant C functions in the Perl code base.

In reply to 3-arg open() does not give warnings!? by graff

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