Like you said there will be a warning but the pattern will match files. It matches files starting with 'd6' with any number of characters after it. I imagine the next question after fixing the file open issue would have been 'why are only some files being opened?'

use warnings;
use strict;
use v5.10;

say foreach glob("\d{6}*");

__END__
Unrecognized escape \d passed through at globmeta.pl line 5.
d60000000000b.txt
d600.txt
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