foreach key
  foreach dependency of that key
   if this dependency is the one I want to delete, delete this key
  continue looping
 continue looping

##</code><code>##

  if( grep { $_ eq $key } @{$dependencies{$key}} ) {
    print "Going to call sub for $key\n";
    mydel($key);
  }

##</code><code>##

my %requiredby;
for my $key (keys $dependencies) {
	for my $dep (@{$dependencies{$key}}) {
		push @{$requiredby{$dep}}, $key;
	}
}

# Or alternately, using a statement modifier to be shorter
my %requiredby;
for my $key (keys %dependencies) {
	push @{$requiredby{$_}}, $key for @{$dependencies{$key}};
}

##</code><code>##

#!/usr/bin/env perl5

use strict;
use warnings;

my %hash = ( 'one' => 1, 'two' => 2, 'three' => 3, 'four' => 4, 'five' => 5);
my %dependencies = ( 'one' => [], 'two' => [], 'three' => ['four'],
                     'four' => ['one','two'], 'five' => ['three'] );

my %requiredby;
for my $key (keys %dependencies) {
	push @{$requiredby{$_}}, $key for @{$dependencies{$key}};
}

sub my_del {
	my $thiskey = shift;

	# Find all the keys that depend on us, and so have to go
	for my $del ( @{$requiredby{$thiskey}} ) {
		# Recurse and delete one key that requires us
		print "Going to call sub for $del\n";
		my_del($del);
	}

	# And finally get rid of us
	delete $hash{$thiskey};
	print "Deleting $thiskey\n";
}


my_del('one');

##</code><code>##

% ./tst.pl 
Going to call sub for four
Going to call sub for three
Going to call sub for five
Deleting five
Deleting three
Deleting four
Deleting one