I have noticed that many file IO operations, such as read(), work on in-memory string filehandles, but sysread() doesn't. So, how can I tell if someone has passed me filehandle to such a thing, so I can work around this limitation?

#!/usr/bin/perl
use strict;
use warnings;
sub dbg {
   my ($op, $fh, $rc, $scalar) = @_;
   $rc = "UNDEF" unless defined $rc;
   print "<$op><$rc><$scalar>\n";
   close $fh or die;
}
sub doit_read {
   my ($fh) = @_;
   my $rc = read $fh, my $scalar, 5;
   dbg "read", $fh, $rc, $scalar;
}
sub doit_sysread {
   my ($fh) = @_;
   my $rc = sysread $fh, my $scalar, 5;
   dbg "sysread", $fh, $rc, $scalar;
}
my $fh;
open $fh, "<", \"/etc/passwd" or die; doit_sysread $fh;
open $fh, "<", \"/etc/passwd" or die; doit_read $fh;
open $fh, "<",  "/etc/passwd" or die; doit_sysread $fh;
open $fh, "<",  "/etc/passwd" or die; doit_read $fh;
[download]

Output:

<sysread><UNDEF><>
<read><5></etc/>
<sysread><5><jrw32>
<read><5><jrw32>
[download]