You may need to restructure how you run your script a bit to do this, but you could start by parsing the output of gunzip -l to get the uncompressed size. Then you can keep track of how much data you have read, and use that to update your progress bar. E.g. something like:

my $amount_so_far = 0;
while (my $line = <>) {
   $amount_so_far += length($_);
   update_progess_bar_as_needed($amount_so_far,$total_length);
}
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Of course, you need to get the $total_length into your script, which you could do either by passing in the filename to your prog.pl and having it call gzip (both with -l to get the length and with -dc to do the unzipping), or by externally calling gunzip -l to get the length and passing that in as a parameter to prog.pl. I would recommend the former, BTW...

Good luck...