if (-e '/path/to/somefile') #method one (perldoc -f -X)
{
  #method two (die statements, see perldoc -f open)
  open(OUTFILE, ">>/path/to/somefile") or die "can't open file: $!";
  #or
  open(INFILE, "/path/to/somefile") or die "can't open file: $!";
}