#!/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)