eval {
  local $SIG{ALRM} = sub { die "too long\n" };
  alarm 30; #seconds
  system( '/usr/local/bin/scp', 'me@example.com:foo.txt', '.' );
  alarm 0;
};
if ($@ eq "too long\n") {
  # time out
}
elsif ($@) {
  #something else went wrong
}
else {
  # ok
}