Then you must be using it wrong. Show us how you used the solution posted.

I am confident that s/_$// works for the sample data you posted. Show us your code.

Hi finddata
As suggested by davido Please check below it works,


#!/usr/local/bin/perl
use strict;
use warnings;

my $str = 'DCMS_DEMO_new_block2_checklist_tmp_rev1_';
chop $str;

print "$str\n";

my $regstr = 'DCMS_DEMO_new_block2_checklist_tmp_rev1_';
$regstr =~ s/_$//;

print $regstr;
[download]


OUTPUT:
DCMS_DEMO_new_block2_checklist_tmp_rev1
DCMS_DEMO_new_block2_checklist_tmp_rev1

Also for your current input, a chop will also work (which is not recommended if your input line does not have _ always at end.It will remove the last character blindly)
As other monks suggested, please try first and post the code which is not working instead of expecting the full code by other monks here.

sub ir($$);
sub ir($$) {
    my $prefix = shift @_;
    $prefix=~ s/_$//;
    print $prefix,"\n";
    my $dir = shift @_;
    print "&&&$dir&&&","\n";
    opendir(DIR, $dir) or die $!;
    my @entries = readdir(DIR);
    print "****@entries***";
    close(DIR);
    foreach my $file (@entries) {
        next if ($file =~ /^\.+$/);
        if ( -d $dir . '/' . $file) {
            ir($prefix . $file .'_', $dir . '/' . $file);

        } elsif ( ( -f $dir . '/' . $file ) && ( $file =~ /\.config$/ 
+) && ($file !~ /^$prefix/)) {
            my $suffix = $file;

            $suffix=~s{\A[^.]*}{}xms;
        print "^^^$suffix^^^","\n";
                   #    print $suffix,"\n";
            rename $dir . '/' . $file, $dir . '/' . $prefix . $suffix 
+;
        }
    }
}
ir('',$output_dir);


here print prefix has underscore at the end of the line
[download]

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]