while ( $line =~ /\controller_name,\s+(\w+)/g ) {
     my $fourth = $1;

print "$fourth";

  }
[download]

Also, this loop is rather suspect (and not just because of the rather bizarre indentation). Why not just use an if-conditional to do the extraction and printing? Then you won't be forced to use the /g modifier to keep the while-loop from running forever.

Win8 Strawberry 5.8.9.5 (32)  Sat 05/22/2021 11:10:41
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings
my $line = "controller_name,bist_top_ctrl\n";

if ($line =~ /controller_name,\s*(\w+)/) {
    my $fourth = $1;
    print "'$fourth' \n";
    }
^Z
'bist_top_ctrl'
[download]