By design, glob only checks for actual existence of files if you include * or ? in that part of the path:

$ echo hi there>foo

$ ls
foo

$ perl -wle'print for glob "{a,b,c,}fo{*,}"'
afo
bfo
cfo
foo
fo
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This pattern expands to {afo,bfo,cfo,fo,afo*,bfo*,cfo*,fo*}. Only the subpatterns containing a ? or * will be restricted to what exists. In the simple case, glob "joe.txt" will always return joe.txt.