Just to add some more examples, here are 5.8 (which looks like what I would expect) thru 5.14. All are Strawberries except ActiveState 5.8.9. So it looks, as mentioned above, like something got b0rked in 5.10 and fixed in 5.14 (correct again). 

c:\@Work\Perl>perl -wMstrict -le
"print $];
 my $s = 'alter string-N-U-M-B-E-R-O-N-E and string-N-U-M-B-E-R-T-W-O.
+';
 $s =~ s/(?=\S+([[:upper:]]-){2,}[[:upper:]]\b)/%/g;
 print qq{'$s'};
"
5.008009
'alter %s%t%r%i%n%g%-%N%-%U%-%M%-%B%-%E%-%R%-O-N-E and %s%t%r%i%n%g%-%
+N%-%U%-%M%-%B%-%E%-%R%-T-W-O.'

5.010001
'alter %string-N-U-M-B-E-R-O-N-E and %string-N-U-M-B-E-R-T-W-O.'

5.012003
'alter %string-N-U-M-B-E-R-O-N-E and %string-N-U-M-B-E-R-T-W-O.'

5.014004
'alter %s%t%r%i%n%g%-%N%-%U%-%M%-%B%-%E%-%R%-O-N-E and %s%t%r%i%n%g%-%
+N%-%U%-%M%-%B%-%E%-%R%-T-W-O.'

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In reply to Re: look-ahead greed changed between perl releases? by AnomalousMonk
in thread look-ahead greed changed between perl releases? by raygun

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