If I understand correctly, and ignoring the reverse line stuff, what you want to extract is everything between the last occurrence of 'workset((..' up to 'dIonly', and that there are a number of these in each line. If that is the case I would forget about reversing and just split each line on 'dIonly', then find the substring for each segment:

my @a_out;

my $line = read_file('data.txt');

if ($line =~ /\bdIonly\b/) {
    # remove everything after last 'dIonly'
    $line =~ s/(.*)\bdIonly\b.*?$/$1/;
    my @segments = split(/\bdIonly\b/,$line);
    for my $str (@segments) {
        if ($str =~ /.*(\bworkset\b\(\(.*)/ ) {  
            my $workset = $1;
            print "workset content: $workset\n\n";
            push(@a_out,$workset);
        }
        else {
            print "no workset\n";
        }
    }
}
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