Imagine Perl had the ability to die to a labeled eval so that when
you write

  FOO: eval {...}

you could die to that labeled eval

  die FOO "bar";

That is essentially what Devel::Unwind gives you.
Two custom keywords 'mark','unwind' are added allowing
you two write

  use Devel::Unwind;

  mark FOO {...} or do {...}
  unwind FOO "bar";
  unwind FOO "bar","baz";
  unwind FOO (bless [], "Bar");
  unwind FOO;

Wherever you would put a block 'eval' an 'mark' expression can be
used.  And wherever you would 'die' you can 'unwind'. If a
$SIG{__DIE__} handler is installed then it gets called on
'unwind'. The arguments to 'unwind' are treated the same way as the
arguments to 'die'. Multiple arguments are joined to together, a single
argument is passed through untouched unless it is a object with
PROPAGATE method in which case $@ gets replaced by the return value of
that method. For details read the documentation of die.