$!      If used numerically, yields the current value of the C "errno"
               variable, or in other words, if a system or library call fails,
               it sets this variable.  This means that the value of $! is
               meaningful only immediately after a failure:

sub do_open {
  # ./datei gibt es nicht
  open my $FH,'<','./datei' or die $!;
}

.....

eval {
  do_open()
} or die "Fehler: $@";