This was originally noticed with Octave when using the help command, or when entering a command with invalid or missing arguments.
Warnings like this appear before the actual command output begins:
Unescaped left brace in regex is
deprecated here (and will be fatal in Perl 5.32), passed through in
regex; marked by <-- HERE in m/^\s+@([[:alnum:]][[:alnum:]\-]*)({
<-- HERE })?\s*/ at /usr/share/texinfo/Texinfo/Parser.pm line 5481.
Unescaped left brace in regex is deprecated here (and will be fatal in
Perl 5.32), passed through in regex; marked by <-- HERE in
m/^\s+@([[:alnum:]][[:alnum:]\-]*)({ <-- HERE
})?\s*(\@(c|comment)((\@|\s+).*)?)?/ at
/usr/share/texinfo/Texinfo/Parser.pm line 5485.
Tested with Octave 4.4.1 on Arch Linux x86-64 with Perl 5.28.0.
At least Debian and Red Hat maintainers have patched this but I don't know if this has been reported to Texinfo developers.