If you do this

my $file = "/full/path/to/file.txt";
open(FILE, "<", $file) or die "Whoops: $!";
do stuff
close(FILE);
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and /full/path/to/file.txt exists, you have r to the file and x to each directory in the path than it will work.

If it does not work then there is a permissions issue most likely. (or the file doesn't exist) either way the die will tell you what failed.