I was also confused about what this $$blk_ref was about. And I just punted that issue in my direct post. It would be helpful if the OP showed more of his application. Dereferencing a ref to a single scalar is a relatively rare thing in Perl. That is because Perl array iterator operations are very good at hiding this nastiness.

For fun, I used your example data and coded the loop a couple of different ways. Neither of which use an explicit dereferencing operation.

use strict;
use warnings;
use Data::Dump qw(dd);

my @strings = (
   'SMS,SMS1,20190811',
   'SMS,SMSh,20190811',
   'SMS,SMSH,20190811',
   'SMS,SMSx,20190811',
   'SMS,SMSi,20190811',
   'SMS,SMSX,20190811',
   'SMS,SMSI,20190811',
   );

# map{} is a logical thought for an array transformation
#
# Could assign back to @strings or can make 
#   a new array, @strings2
# Could use an "if" and concatenate a message if true
#   and return $_ in any event. Ternary operator here
#   gives a place to put a single token that is 
#   not "SMSblk"

my @strings2 = map{/SMS[1HI]/i ? "$_ is SMSblk":$_}@strings;  
 
dd \@strings2;

=prints:
[
  "SMS,SMS1,20190811 is SMSblk",
  "SMS,SMSh,20190811 is SMSblk",
  "SMS,SMSH,20190811 is SMSblk",
  "SMS,SMSx,20190811",
  "SMS,SMSi,20190811 is SMSblk",
  "SMS,SMSX,20190811",
  "SMS,SMSI,20190811 is SMSblk",
]
=cut

# Or with a for loop instead of map{} 
#   to modify original array:
# Foreach creates an alias and modifying that
#   alias modifies the original array
# No tricky dereferencing is needed.

foreach (@strings)
{
   $_ .= " is SMSblk" if /SMS[1HI]/i;
}

dd \@strings;

=prints:
[
  "SMS,SMS1,20190811 is SMSblk",
  "SMS,SMSh,20190811 is SMSblk",
  "SMS,SMSH,20190811 is SMSblk",
  "SMS,SMSx,20190811",
  "SMS,SMSi,20190811 is SMSblk",
  "SMS,SMSX,20190811",
  "SMS,SMSI,20190811 is SMSblk",
]
=cut
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