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";
}
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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.