>perl -wMstrict -le
"my @config = ([ 'fee fie' ], [ 'foe'], [ 'fum' ],);
 printf qq{'$_->[0]' } for @config;  print '';
 ;;
 @config = map { $_->[0] =~ s{(f[eio]e)}{\u$1}xmsg;  $_; } @config;
 printf qq{'$_->[0]' } for @config;  print '';
 ;;
 map { $_->[0] =~ s{ (F [eio] e) }{\U$1}xmsg } @config;
 printf qq{'$_->[0]' } for @config;  print '';
 ;;
 for my $ar (@config) {
   $ar->[0] =~ s{ (F [EIO] E) }{\L$1}xmsg;;
   }
 printf qq{'$_->[0]' } for @config;  print '';
"
'fee fie' 'foe' 'fum'
'Fee Fie' 'Foe' 'fum'
'FEE FIE' 'FOE' 'fum'
'fee fie' 'foe' 'fum'