Just to drive davido's point home:

c:\@Work\Perl\monks>perl -wMstrict -le
"for my $s (qw(
   DCMS_DEMO_
   DCMS_DEMO_new_block2_
   DCMS_DEMO_new_block2_checklist_tmp_
   DCMS_DEMO_new_block2_checklist_tmp_rev1_
   )) {
   my $subject = $s;
   print qq{'$subject'};
   $subject =~ s/_(?=[^_]*$)//;
   print qq{'$subject' \n};
   }
"
'DCMS_DEMO_'
'DCMS_DEMO'

'DCMS_DEMO_new_block2_'
'DCMS_DEMO_new_block2'

'DCMS_DEMO_new_block2_checklist_tmp_'
'DCMS_DEMO_new_block2_checklist_tmp'

'DCMS_DEMO_new_block2_checklist_tmp_rev1_'
'DCMS_DEMO_new_block2_checklist_tmp_rev1'
[download]

That's with your original regex, and I get the same output with  s/_$// also.