This prints test. Changed the lookahead to ((?:(?!\n\n).)*)

use strict;
use warnings;

my $s = do {local $/, <DATA>};

$s =~ s/\w+\s+\w+\s*\d+\s*\,((?:(?!\n\n).)*)\n\n/'test'/egsi; 

print $s;

__DATA__
LMTRILOG     db 5 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 
+, 0 , 0 ,
0;
             db 25 , 1 , 1 , 1 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0
+ , 0 , 0 ,
 0;
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