#!/usr/bin/perl -lw
my %hash = qw(a b c d d c b a);

foreach my $key (%hash) {
  print $key;
  delete $hash{$key};
}
# output:
# c
#
# a
# Segmentation fault (core dumped)

foreach my $key (%hash) {
  print $key;
  delete $hash{"$key"};
}
# same output with seg fault

foreach my $key ( @{[%hash]} ) {
  print $key;
  delete $hash{$key};
}
# output:
# c
# d
# a
# b
# b
# a
# d
# c