Bash: Unescaped Left Brace in Regex

A strange message was recently received from a Bash script running under Linux Mint 18:

Unescaped left brace in regex is deprecated, passed through in regex; marked by <-- HERE in m/%{ <-- HERE (.*?)}/ at /usr/bin/print line 528.
Error: no such file "test\n"

Slightly confusing, as it reads like a Perl error, rather than bash. Below is another variety of the same thing

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