What dave_the_m wrote is correct, as well as being a great example of using perldoc to read the Perl documentation.  (I use it so much on Linux that I've got "pd" aliased to "perldoc", and "pf" to "perldoc -f").

However, it looks like you are trying to open a directory ("/tmp") rather than a file.   If that's the case, you can still use a filehandle (I like to create it with FileHandle), but you probably want "readdir" rather than "open":

    use FileHandle;
    my $fh = new FileHandle;
    my $dir = "/tmp";
    opendir($fh, $dir) or die "Unable to open directory '$dir' ($!)\n"
+;
    my @files = readdir($fh);
    closedir $fh;
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At the end of this segment of code, the list "@files" will contain the files from the directory "/tmp".  Note that the names will NOT be full pathnames, so if you want that, you'll have to prepend "/tmp" to each member in the list:

    map { $_ = "$dir/$_" } @files;    # Prepend "/tmp" to all files
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Another note -- the directories "." and ".." will also be in the list, and you usually want to skip them for any further processing.

I'll let you try "perldoc -f opendir" and "perldoc -f readdir" if you want to read more about them.