>perl -wMstrict -le
"unshift @ARGV,
   'not foo camelCase NOT BAR namesCanBeDifferent oK Not Baz';
 ;;
 for (@ARGV) {
   print qq{'$_'};
   s{ ([[:lower:]]*) ((?<=[[:lower:]]) [[:upper:]][[:lower:]]*) }
    {\U$1_$2}xmsg;
   print qq{'$_' \n};
   }
"
"aB aB aB"  " aB "  "aBc aBc aBc"  " aBc "
"No No No"  " No "  "Not Not Not"  " Not "
'not foo camelCase NOT BAR namesCanBeDifferent oK Not Baz'
'not foo CAMEL_CASE NOT BAR NAMES_CAN_BE_DIFFERENT O_K Not Baz'

'aB aB aB'
'A_B A_B A_B'

' aB '
' A_B '

'aBc aBc aBc'
'A_BC A_BC A_BC'

' aBc '
' A_BC '

'No No No'
'No No No'

' No '
' No '

'Not Not Not'
'Not Not Not'

' Not '
' Not '