c:\@Work\Perl\monks>perl -wMstrict -le
"use Data::Dump qw(dd);
 ;;
 my $s = 'ab THE c RAIN def IN g SPAIN hi FALLS jk MAINLY l ON m';
 print qq{$s};
 ;;
 my @off_len;
 while ($s =~ m{ [[:upper:]]+ }xmsg) {
   push @off_len, [ $-[0], $+[0]-$-[0] ];
   }
 dd \@off_len;
 ;;
 my @repl = qw(FOURSCORE AND 7 YEARS AGO OUR FATHERS XXX YYY);
 ;;
 my $least_i = $#off_len < $#repl ? $#off_len : $#repl;
 for my $rli (reverse 0 .. $least_i) {
   my ($off, $len) = @{ $off_len[$rli] };
   substr $s, $off, $len, $repl[$rli];
   }
 print qq{$s};
"
ab THE c RAIN def IN g SPAIN hi FALLS jk MAINLY l ON m
[[3, 3], [9, 4], [18, 2], [23, 5], [32, 5], [41, 6], [50, 2]]

ab FOURSCORE c AND def 7 g YEARS hi AGO jk OUR l FATHERS m