% 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)

##</code><code>##

#!/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)