my $output = qx(powercfg -l);

## You've got your output loaded into the $output variable. 
## Still want to write it to a file?
open my $OUTPUT,'>','output.txt' or die "Couldn't open output.txt: $!\n";
print $OUTPUT $output;
close $OUTPUT

## Now you can loop through each line and
##   parse the $line variable to extract the info you are looking for.
foreach my $line (split /[\r\n]+/,$output) {
  ## Regular expression magic to grab what you want
}