I would guess there's a special case when the brace is the first character in the regex.

$ perl -wE 'say $]; say "Match literal" if "{,3}" =~ m<{,3}> && "x" !~
+ m<{,3}>;'
5.030001
Match literal

$ perl -wE 'say $]; say "Match space" if " {,3}" =~ m< {,3}> && "x" !~
+ m< {,3}>;'
Unescaped left brace in regex is illegal here in regex; marked by <-- 
+HERE in m/ { <-- HERE ,3}/ at -e line 1.
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