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.