An (efficient) alternative.

use strict;
use warnings;
use File::Spec::Functions qw/ canonpath /;


my $fullpath = '/www/stage/sites/MyCompany/docs/systems/./foo/../docum
+entation///';
my $sitedoc = '/www/stage/sites/MyCompany/docs/..';
my $want = 'config.cfg';


print find_file($fullpath, $sitedoc, $want) ? "found" : "not found";


sub find_file
{
    my ($path, $root, $want) = @_;

    $path = canonpath($path);
    $root = canonpath("$root/..");

    my $found;
    while ($path ne $root) {
        $found = 1, last if -f "$path/$want";
        $path =~ s!\\[^\\]+$!!;
    }

    return $found;
}
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