From the perlvar documentation:

   $OSNAME
       $^O     The name of the operating system under which this copy of Perl
               was built, as determined during the configuration process.  The
               value is identical to $Config{'osname'}.  See also Config and
               the -V command-line switch documented in perlrun.      

Of course you could also just check and see which of the directories you are interested in exists, rather than worrying about what the operating system is.

Use the Config module. Sometimes $^O doesn't give you all the details:

C:\>perl -MConfig -e "print $Config{osname},\"\n\";"
MSWin32
[download]

'perldoc perlvar'

$OSNAME
$^O     The name of the operating system under which this copy of Perl
        was built, as determined during the configuration process. The
        value is identical to $Config{'osname'}. See also Config and t
+he
        -V command-line switch documented in perlrun.
[download]

Defining Your Needs

If you know that your module simply will not run in a certain environment, you should set up prerequisites. These allow you to provide a level of safety for your users. Prerequisites include:

OSes that your module will not run under

Check $^O and %Config for this. $^O will tell you the name of the operating system. Sometimes, this isn't specific enough, so you can check %Config.

use Config;

if ( $Config{ osname } ne 'solaris' || $Config{ osver } < 2.9 )
{
    die "This module needs Solaris 2.9 or higher to run.\n";
}
[download]

Source: http://www.perl.com/lpt/a/2005/04/14/cpan_guidelines.html

Alceu Rodrigues de Freitas Junior
---------------------------------
"You have enemies? Good. That means you've stood up for something, sometime in your life." - Sir Winston Churchill

jdporter fixed markup

Another option is to forget OS detection and just look for the files.

open (FH, 'c:/snmpauto') || 
# I don't know Windows so this path syntax may be off
open (FH, '/root/snmpauto') || 
die (q(Couldn't open the file!));
[download]

Or ask the user where snmpauto resides

That would require trusting the user. From the locations where he's chosen to put these files, I take it only the system administrator is supposed to be able to alter the contents of them. (I still would have put it under /etc rather than /root though.)

Why not simply have a list of possible locations the file is allowed to reside, and check them in a well-defined (and documented) order until you hit one? This is a good idea because it won't require any additional logic if you choose to support another OS (with yet another path). Example:

# I assume 'snmpauto' is the name of a file.
use File::Spec;
my $snmp_file = find_snmpauto();

sub find_snmpauto {
    my $filename = 'snmpauto';
    my @paths = qw(c:\\ /root);

    for (@paths) {
        my $name = File::Spec::catfile($_,$filename);
        return $name if -f $name;
    }

    # we hit here only if no file was found.
    die "Can't find a file '$filename' in the search path (path includ
+es:"
       .join(', ',@paths)
       .")\n";
}
[download]

This will result in the first filename you find to be correct being stored in $snmp_file; it will die (with a list of acceptable paths) if it fails. This way, if you add a new location, you only have to add to the @paths array.