For example:

 #!/usr/local/bin/perl -w

%hash = (foo => 42, bar => 23);
lock_keys(%hash);

# Now, try to set %hash{'something'}
while(my($key, $value) = each %hash) {
    print "$key -> $value\n";
}

unlock_keys(%hash);

You can lock specific values as follows:

#!/usr/local/bin/perl -w

%hash = (foo => 42, bar => 23);
lock_key(%hash, 'foo');

# Now, try to set 'foo' and see what happens

unlock_keys(%hash, 'foo');

8.118. Getopt::Std		8.120. I18N::Collate