open(FILE, "<generic_bist_ctrl_defines.sv") || die "File not found";
my @lines = <FILE>;
close(FILE);

foreach(@lines) {
   $_ =~ s/=131/= $a/g;
}

open(FILE, ">generic_bist_ctrl_defines.sv") || die "File not found";
print FILE @lines;
close(FILE);

close $in;