jrw:

You could do something like this:

sub is_FH_a_wrapped_scalar {
    my $FH = shift;
    return grep { $_ eq "scalar} } PerlIO::get_layers($FH);
}
[download]

This uses PerlIO to find the layers of I/O handlers on your filehandle. If one of them is named "scalar", then the filehandle is ultimately wrapping a scalar variable.

Unfortunately, this is the only way I've found to do it, and PerlIO might not be compiled in all versions you use.

$ perl pm_1214805.pl
FH is a wrapped scalar, do something else!
<read><5></etc/>
<sysread><5><SYSTE>
<read><5><SYSTE>

$ cat pm_1214805.pl
#!/usr/bin/perl
use strict;
use warnings;

sub is_fh_a_wrapped_scalar {
    my $fh = shift;
    my @layers = PerlIO::get_layers($fh);
    return grep { $_ eq "scalar" } @layers;
}

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 ($fh) = @_;
   if (is_fh_a_wrapped_scalar($fh)) {
       print "FH is a wrapped scalar, do something else!\n";
   }
   else {
       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]

...roboticus

When your only tool is a hammer, all problems look like your thumb.