Maybe something like (untested):

use Cwd;
my $docroot = '/www/stage/sites/MyCompany/docs';
my $cfgdir  = $docroot;
my $dir = getcwd;
$dir =~ s{^$docroot/}{};
my (@dirs) = split '/', $dir;
while (not -e "$cfgdir/filename.cfg" and @dirs) {
    $cfgdir .= '/' . shift(@dirs);
}
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?