The following code shows a function that will receive the directory where to look for the files and the filename you got from function (TeST.maK). It will then read the directory contents and perform a test to see if the filename matches any of the files in the directory. The i modifier on the regexp makes that match case insensitive. If a match is found, the actual name of the file is returned, otherwise the returned value will be an empty string.

sub getFileName {
    my $dir  = shift;
    my $file = shift;

    opendir(DIR, $dir);
    my @dirContent = readdir(DIR);
    closedir(DIR);

    foreach my $content (@dirContent) {
        if($file =~ /^$content$/i) {
            # filename matches
            return $content;
        }
    }
    return '';
}
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note: verbose solution for clarity.