I'm not sure why you think you need to slurp the file:

use strict;
use warnings;
use 5.010;
 
my $str =<<'ENDOFSTRING';
       unit 53 {
            description 1234-rwan-1;
            vci x.yy;
        }
        unit 54 {
            description 45ff-rwan-1;
            vci x.yz;
ENDOFSTRING
 

open(my $INPUT, '<', \$str)
  or die "Couldn't open string for IO: $!";
  
while (my $line = <$INPUT>) {
  if ($line =~ /^
               \s+ description \s
               (\w{4})
               (- \w+ - \w+);
              /xms 
  ){
      say "$1 <=> $2";
  }
}

--output:--
1234 <=> -rwan-1
45ff <=> -rwan-1
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