if ($str =~ /(\\x{[A-F\d]+})/i){    // error by perl -Tcw

 if ($str =~ /^(\d{2,4})[^\d](\d{2})/){     // no error by perl -Tcw

##</code><code>##

DB<4> $str =  '\\x{A3f4}'                                           # literal curly

DB<5> if ( $str =~ /(\\x{[A-F\d]+})/i ) { print $1 }                # implicit but deprecated                                            
\x{A3f4}     

DB<6> use warnings; if ( $str =~ /(\\x\{[A-F\d]+})/i ) { print $1 } # explicit and no warning 
\x{A3f4}     

DB<7> $str =  '123X12'                                              # no curlies, just repeated numbers

DB<8> if ( $str =~ /^(\d{2,4})[^\d](\d{2})/) { print "$1;$2" }      # meta curly
123;12