I'd try something like the following:

use File::Spec;

# define variables as above

opendir(CURRDIR, $mypath) or die "Can't open $mypath: $!";
my @files = map { File::Spec->catfile($mypath, $_) }
        grep { /^[\d\w\s]+\.$filetype/ }
        readdir CURRDIR;

foreach my $file (@files) {
    # reveal potentially nasty leading/trailing whitespace
    print "Processing ->$file<-\n";
    my $book = $Excel->Workbooks->Open($file) 
        or die "Error opening $file: $!";
    $book->Close();
}
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This lets a tested, cross-platform module File::Spec handle the dirty business, avoids string interpolation that may be trouble, and is slightly more Perlish.