c:\@Work\Perl>perl -wMstrict -le
"my $s = 'xxx aBc yyy dE zzz';
 print qq{\$s: '$s'};
 ;;
 print qq{\$1: '$1'} if $s =~ s{ [AaBbCcDdEe]+ }{XXX}xmsg;
 print qq{\$s: '$s'};
"
$s: 'xxx aBc yyy dE zzz'
Use of uninitialized value $1 in concatenation (.) or string at -e line 1.
$1: ''
$s: 'xxx XXX yyy XXX zzz'