Turning your loop into recursion might look like this:

#!/usr/bin/perl
use strict;

my $exception;

$exception->{a}{b}{c}{d} = 4;

walk_hash( $exception );

sub walk_hash {
    my $arg = shift;

    foreach my $key ( keys %{ $arg } ) {
        my $value = $arg->{ $key };
        if ( ref( $value ) eq 'HASH' ) {
            walk_hash( $value );
        }
        else {
            print "\t$key : $value\n";
        }
    }
}
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So in the sub, check the type of the value. If it's a hash, walk it, otherwise print the key:value. You can pass other things to the recursion if you need them, like the depth so you can alter indentation.

Phil