I was not aware Windows reports incorrect filesizes. Then you have to rely on copy's reporting. Use this pipe-open form instead:

use strict;
use warnings;
my $cmd = 'copy.exe 'from', 'to', '/z' |';
open(EXE, $cmd) || die "$!";
while(<EXE>){
        print "read from program output: ".$_."\n";
}
print "done.\n";
close(EXE);
[download]

Or use IPC::Run or any other IPC::* module mentioned here or there.

Hello

As for code it wont work. i tried earlier on and it gives output only when its done but i need to monitor progress

As for IPC ill check

This works for me on Windows:

use strict;
use warnings;
use feature 'say';

use POSIX ":sys_wait_h";
use Capture::Tiny ':all';

my $cmd = 'cmd /c copy a1GBfile a1GBfile_copy /z';
my $out = IO::File-> new_tmpfile;

capture_stdout {
    my $pid = system( 1, $cmd );

    while () {
        last if waitpid( $pid, WNOHANG );
        
        undef local $/;
        seek $out, 0, 0;
        my $s = <$out>;
        next unless $s and $s =~ m/.*\D(\d+)%/s;

        say STDERR "progress so far: $1 percent";
        
        sleep 1;
    }
} stdout => $out;

close $out;

__END__

progress so far: 0 percent
progress so far: 60 percent
progress so far: 70 percent
progress so far: 80 percent
progress so far: 89 percent
progress so far: 99 percent
[download]

I'd prefer to use filehandle opened on Perl scalar, but Capture::Tiny isn't happy about the idea. + You can tee_stdout instead of capture_stdout, to see "normal" output in console, and replace say STDERR ... line with real logic of what you trying to do. Or, to see if something is still happening, just check -s $out for change instead of slurping.

You want to say that from cmd.exe copy 'from' 'to' /z reports on progress as it does the copy but when run via Perl's pipe-open it does not update the output until it finishes? In some OSs I find my luck runnning out faster than in others ...