If that's the way things are, then maybe:

>perl -wMstrict -le
"my $str = 'eternal corruption defilement';
 ;;
 $str =~ s{ \b ([[:alpha:]]) [[:lower:]]* \s* }{\U$1}xmsg;
 ;;
 print qq{'$str'};
"
'ECD'
[download]

Update: Removed an unnecessary level of capture grouping in regex above. Also:  [[:lower:]]* might be better as  [[:alpha:]]* instead.

Update: Or even:

>perl -wMstrict -le
"my $str = 'eternal corruption defilement';
 ;;
 $str =~ s{ (\b [[:alpha:]]+ \s*) }{ uc substr $1, 0, 1 }xmsge;
 ;;
 print qq{'$str'};
"
'ECD'
[download]