gistfile2.pl

# Set up the hook
my $exit_handler = sub {
    print "Bang!\n";
};

#And install it
BEGIN {
    *CORE::GLOBAL::exit = sub { $exit_handler->(@_) };
}

# Do code that we don't really want to exit
exit 1;

# The closest thing to an un-override we can do
$exit_handler = sub {
  CORE::exit $_[0];
};

# This will exit for real
exit 2;

oexit-user.pl

use Devel::OverrideExit;
use Test::More tests => 1;

{
  my $exits = 0;
  my $guard = Devel::OverrideExit->new(sub {
      $exits++;
    }
  );

  # Standin for the code that should exit:
  exit 1;

  is($exits, 1, "Called exit once");
}

OverrideExit.pm

package Devel::OverrideExit;
use strict;
use warnings;

my $exit_handler = sub {
  CORE::exit $_[0];
};

BEGIN {
  *CORE::GLOBAL::exit = sub { $exit_handler->(@_) };
}

sub new {
  my ($class, $exit) = @_;

  my $self = bless { saved => $exit_handler }, $class;
  $exit_handler = $exit;
  return $self;
}

sub DESTROY {
  $exit_handler = shift->{saved};
}

1;