It is Perl reporting an OS message. Consided open F, $foo or die $! say you get permission denied, that is an OS message delivered via perl.

You don't have to use real file descriptors. You could simply open a DBM database or a tied hash, pretend that the keys are the file descriptors and just append to the values. It actually simplifies the code, but it will be a big speed hit.

Here is one of the examples from Re: Binary file handling converted to use a hash tied to a file.

sub rotate_minus90 {
    my ( $infile, $outfile, $tmp ) = @_;
    $tmp ||= 'c:/tmp/temp';
    open IN, $infile or die "Can't read $infile $!\n";
    # find number of columns and open a temp file for each
    chomp( local $_ = <IN> );
    my @data = split ' ';
    my $num_cols = $#data;
    dbmopen(my %fhs, $tmp, 0666) or die "dbmopen can't grok $tmp $!\n"
+;
    $fhs{$_} = "$data[$_]\t" for 0..$num_cols;
    while( <IN> ) {
        chomp;
        @data = split ' ';
        $fhs{$_} .= "$data[$_]\t" for 0..$num_cols;
    }
    close IN;
    open OUT, ">$outfile" or die "Can't write $outfile $!\n";
    for ( reverse 0.. $num_cols ) {
        print OUT $fhs{$_}, "\n";
    }
    dbmclose(%fhs);
    close OUT;
}
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