use Getopt::Long qw(:config bundling);
GetOptions('input-file|i=s' => \my @file);

my ($stdin_is_redir, $want_stdin) = !! (stat STDIN)[0];
for my $arg (@file) {
    my $fh;
    if ('-' eq $arg) {
        $want_stdin = 1;
        # This is never triggered under cron.
        warn "stdin isn't connected- missing pipe?\n" and next
            unless -p STDIN or ($stdin_is_redir and ! -t STDIN);
        $fh = *STDIN{IO};
    }
    else {
        require Path::Tiny;
        my $file = path($arg);
        $fh = eval { $file->openr } or warn "$file: $@->{err}\n" and next;
    }
    # do_something($_) while <$fh>;
}

# This is always triggered under cron unless $stdin_is_redir
warn "stdin is connected, missing `-i -`?\n"
    if ! $want_stdin and ! -t STDIN and ($stdin_is_redir or -p STDIN);

# Tested with:
# ./script.pl
# ./script.pl -i -
# ./script.pl -i /dev/null
# ./script.pl -i - < /dev/null
# echo test | ./script.pl
# echo test | ./script.pl -i -