The random error is, it's able to open the file (the die action never happens), but unable to read any data ($line is uninitialized)

Since your code isn't hitting the die() after the open() call, the program found the file and had permission to open it. Chances are good you just attempted to read an empty file.

As perlop will explain under the I/O Operators heading, the angled-bracket read returns undef in scalar context when you're at EOF (end-of-file.) If you care about this condition, you should test the definedness of $line before using it, and possibly report EOF as an error condition to the invoker of the script rather than try to use it.

I can reproduce the condition you're seeing by running the code shown at the bottom of this post (which I call y1.pl) with the following test. Note that I'm on a Unix-alike, so you may need to populate an empty file a different way as defined by your OS. Also note that I replace an undefined result with a notice that the line was in fact undefined, using the // operator. This should illustrate the problem I suspect you're having.

Test case:

cp /dev/null file.txt
perl ./y1.pl
echo "hello world" > file.txt
perl ./y1.pl
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y1.pl:

use strict;
use warnings;

my $file_src = 'file.txt';

open(my $fh, '<', $file_src) or die "open() failed: $!";
my $line = <$fh>;
close($fh) or die "close() failed: $!";
printf( "Got a line: %s", $line // '<line-was-undef>' );
[download]

If the writer of this file has it open, it is possible for there not to be anything in it yet, but yet possible for you to have it open for read. One of those reasons could be output buffering. Typically the writer will have to "write" at least 1K before the data gets flushed to the disk.

Perl has some built file test operators, my $size = -s $filename should give you the current file size that you are able to read.

So without fixing the writer to turn off buffering, by say adding $|=1, you could loop for a while, checking $size every second or so (sleep(1) or whatever) for a few minutes. Don't try to read from the open filehandle until $size>0. Usually once you hit "EOF", you can't read past that, but you can delay reading until something is actually available for you to read. That way you won't hit EOF.

Basically this idea just automates the "wait a few minutes" and try again part. Also adding this $size check to the code would I think be of diagnostic value, if you are getting undef read value (EOF) and $size>0, then there is some other problem.

PS: what O/S are you using?

Update: Perl File Test Operators

I tried it and had no such problem (after removing extraneous ')' at end of open statement). perldoc -f open says that if '<' is omitted, it opens for input. Something else must be going on.

Has the other program closed the file before your program starts ?