Just a couple data points for you... On 5.00503 for linux, it doesn't dump. With 5.6.1 it does. However, with warnings on in 5.6.1 I get this: (note, lines 13 & 14 refer to the foreach stanza)

% perl5.6.1 -w test.pl
Use of uninitialized value in substr at test.pl line 14.
Attempt to free unreferenced scalar at test.pl line 13.
Use of uninitialized value in substr at test.pl line 14.
Attempt to free unreferenced scalar at test.pl line 13.

# other invocations....
% perl5.00503 -w test.pl
[nothing]

% perl5.00503 test.pl
[nothing]

% perl5.6.1 test.pl
Segmentation fault (core dumped)
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Update: As crazy noted... a well placed keys() in your foreach loop seems to solve the problem.....

Update 2: It looks like deleting the key before you get to the corresponding value in the foreach loop is causing problems...

#!/usr/bin/perl
use strict;

### show what we're looping over (a,1,b,2,c,3)
my %hash = (a=>1, b=>2, c=>3);
for (%hash) {
  print "OK: $_\n";
}

### check the value *before* we attempt to delete the key
### i.e. 1 before a, 2 before b, 3 before c
%hash = (a=>1, b=>2, c=>3);
for (reverse %hash) {
  substr $_, 0, 1;
  delete($hash{$_}) if /[a-z]/;
  print "Still-OK: $_\n";
}

### emulate your structure where we can potentially delete
### a key first, then loop over the value associated with it
%hash = (a=>1, b=>2, c=>3);
for (%hash) {
  substr $_, 0, 1; # <== segfaults here when testing value associated 
+with deleted key
  delete($hash{$_}) if /[a-z]/;
  print "Not-OK: $_\n";
}

__END__

=head1 OUTPUT
OK: a
OK: 1
OK: b
OK: 2
OK: c
OK: 3

Still-OK: 3
Still-OK: c
Still-OK: 2
Still-OK: b
Still-OK: 1
Still-OK: a

Not-OK: a
Not-OK:
Segmentation fault (core dumped)
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-Blake