use OurCommon;

##</code><code>##

BEGIN {
  $main::SIG{__DIE__} = \&OurCommon::handle_fatal_error; # new line
  ...
  package OurCommon;
}
# outside begin and outside a sub

##</code><code>##

# $lib eq "Math/BigInt/FastCalc.pm" here
eval "use $lib qw/@c/;";