It was the same as using /m on all your match and substitution operators, except it overrode instructions to turn /m off.

For example, qr/foo$/ shouldn't match "foo\nbar\n" even if it's used as /$re/m.

$ perl5010 -le'
   $re = qr/foo$/;
   print $re;
   print "foo\nbar\n" =~ /$re/m ?1:0
'
(?-xism:foo$)   # -m: Turn /m off.
0
[download]

But it did before 5.10

$ perl5008009 -le'
   $re = qr/foo$/;
   print $re;
   print "foo\nbar\n" =~ /$re/m ?1:0
'
(?-xism:foo$)
1               # Matched as if /m was on for the subpattern.
[download]

This was fixed, but there was no way of fixing $* which had the same error.

$ perl5008009 -le'
   $re = qr/foo$/;
   print $re;
   local $* = 1;
   print "foo\nbar\n" =~ /$re/ ?1:0
'
(?-xism:foo$)
1
[download]

So it was dropped.