$ perldoc -f fileno

    fileno FILEHANDLE
            Returns the file descriptor for a filehandle, or undefined
+ if the
            filehandle is not open. If there is no real file descripto
+r at the
            OS level, as can happen with filehandles connected to memo
+ry
            objects via "open" with a reference for the third argument
+, -1 is
            returned.
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Better is:
open my $R, '<', "$file" or die "your specific message $!";
Use a lexical variable, "my" instead of a package global, explicitly say this is for reading (or writing), print an error message and exit your program (i.e. die) if the open does not succeed.

For a disk file, you can assume that all subsequent "prints" to that filehandle will work. There is no need to check if the file handle is still open, it is.

There are boundary cases where the write to a file will fail (like filesystem full) and there are situations where a connection to a network file socket will be lost. Handling those cases are beyond what you are asking about.

Update: While jwkrahn's answer is factually correct, this doesn't have much meaning in practical Perl code. When you open a file handle, you should check that the open worked, and if it did, thereafter you should assume that the file handle is still open. At low level C code, there is such a thing as a fileno. But you will very likely will never ever encounter or need this number in Perl. Just because a file handle is "open", that does not mean that actually using it to read or write will work. At the end of the day, "the proof is in the pudding" - you have to actually use the filehandle in order to know if it indeed works to get or send data.

 $bool = is_FH($fh);
 $bool = is_FH(\*FH);
  This is just a (more intuitively named) alias for is_arg_ok().
  Returns 1 if its argument is an open filehandle.
  Returns 0 if its argument is something other than an open filehandle
+.

 $bool = is_arg_ok($fh);
 $bool = is_arg_ok(\*FH);
  Returns 1 if its argument is an open filehandle.
  Returns 0 if its argument is something other than an open filehandle
+.

 Arguments to the following functions  must be open filehandles. If
 any of those functions receive an argument that is not an open
 filehandle then the function dies with an appropriate error message.
 To ensure that your script won't suffer such a death, you could first
 check by passing the argument to is_FH(). Or you could wrap the
 function call in an eval{} block.

Note that it may be possible that a filehandle opened for writing may
become unwritable - if (eg) the disk becomes full. I don't know how
the below functions would be affected by such an event. I suspect
that they would be unaware of the change ... but I haven't actually
checked.


 $bool = is_R($fh);
 $bool = is_R(\*FH);
  Returns true if the filehandle is readable.
  Else returns false.

 $bool = is_W($fh);
 $bool = is_W(\*FH);
  Returns true if the filehandle is writable.
  Else returns false.

 $bool = is_RO($fh);
 $bool = is_RO(\*FH);
  Returns true if the filehandle is readable but not writable.
  Else returns false.

 $bool = is_WO($fh);
 $bool = is_WO(\*FH);
  Returns true if the filehandle is writable but not readable.
  Else returns false.

 $bool = is_RW($fh);
 $bool = is_RW(\*FH);
  Returns true if the filehandle is both readable and writable.
  Else returns false.

 $bool = is_A($fh);
 $bool = is_A(\*FH);

  Returns true if the filehandle was opened for appending.
  Else returns false.
  Not currently implemented on Win32 with pre-5.6.1 versions of perl (
+and
  dies with appropriate error message if called on such a platform).
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