use IPC::Open3;

sub backticks {
  my $pid = open3(my $wfh, my $rfh, 0, @_)
              or die "open3 failed: $!";
  close($wfh); # assume the script needs no input
  my $output;
  while (<$rfh>) { $output .= $_ }
  close($rfh);
  $output;
}

my $java_output = backticks('java', '--classpath=...', ...);