let's pretend there's an awful lot of them, or the hash is tied, and it's harder to find an existing key than fetch a value
Well, a pretty foolproof method is to build an array of keys to delete, and delete them later. But by what you said I am guessing this list might grow very long, so that might not be feasible?
LanX has a point that resetting the iterator on each loop might result in an endless loop if one is not careful. But I think that resetting the iterator only when keys are actually deleted might be an option, e.g.:
use warnings;
use strict;
use Data::Dump qw/ dd pp /;
my %h = (
1 => [2..7],
8 => [ 9 ],
9 => [ 8 ],
(map { $_ => [ 1 ] } 2..7),
);
dd \%h;
while ( my ($k,$v) = each %h ) {
print "Loop from ",pp($k)," -> ",pp($v),"\n";
rec_delete( \%h, $k );
}
dd \%h;
sub rec_delete {
my ($hash, $key) = @_;
return unless exists $hash->{$key};
my $value = delete $hash->{$key};
keys %$hash;
return unless $value;
print "Recursing ",pp($key)," -> ",pp($value),"\n";
rec_delete( $hash, $_ ) for @$value;
}