If I understand correctly, this hash would be updated with more values as we go down the call stack?

It would be as long as you kept building it up each time you localized it...

#!/usr/local/bin/perl

use strict;
use warnings 'all';

use vars qw(%catchable);

%catchable = ();

sub throw {
    print "in $_[0], deciding if i should throw $_[1]: " .
        (exists $catchable{$_[1]} ? "sure\n" : "nope, won't be caught\
+n");
}

sub foo {
    # try { bar(); baz() } catch foo { ... }
    
    local %catchable = %catchable;
    $catchable{'foo'} = 1; 

    bar();
    baz();
}

sub bar {
    # try { yak(); } catch bar-1 { ... } catch bar-2 { ... }
    
    local %catchable = %catchable;
    $catchable{'bar-1'} = $catchable{'bar-2'} = 1;

    yak();
}

sub baz {
    # do some stufff...
    throw('baz','foo');
    throw('baz','uncaught-exception-1');
}

sub yak {
    throw('yak','foo');
    throw('yak','bar-1');
    throw('yak','bar-2');
    throw('yak','uncaught-exception-2');
}

foo();


__DATA__
bester:~> tmp/monk.pl
in yak, deciding if i should throw foo: sure
in yak, deciding if i should throw bar-1: sure
in yak, deciding if i should throw bar-2: sure
in yak, deciding if i should throw uncaught-exception-2: nope, won't b
+e caught
in baz, deciding if i should throw foo: sure
in baz, deciding if i should throw uncaught-exception-1: nope, won't b
+e caught
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