You can achieve the same effect as Readonly for hashes, without the overhead, using Hash::Util:

#! perl -slw
use strict;
use Hash::Util qw[ lock_hash ];
use constant FOO => { x => 0 };
lock_hash( %{ FOO() } );

++ FOO->{x};

print FOO->{x};
__END__
C:\test>junk7
Modification of a read-only value attempted at C:\test\junk7.pl line 7
+.
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It would be a nice addition to constant if it would (or could be instructed to) do that for you.

Internals::SetReadOnly(), also works to an extent on both hashes and arrays:

#! perl -slw
use strict;
use Internals qw[ SetReadOnly ];

use constant BAR => { x => 1 };
SetReadOnly( BAR );
BAR->{y} = 12345; ## Modification of a read-only value attempted ...

use constant BAZ => [ 1, 2, 3, 4, 5, ];
SetReadOnly( BAZ );
push @{ +BAZ }, 1; # Modification of a read-only value attempted ... 

## Unfortunately, these do not produce errors?
++BAR->{x};
++BAZ->[0];

__END__
C:\test>junk7
Modification of a read-only value attempted at C:\test\junk7.pl line 1
+3.
[download]

I frequently have constant data structures. Having to loop over all the data in my constant just to set the readonly flag sucks and mostly I don't do it. I'd really just wish for constant.pm to learn to do the right thing. I suppose I should just go patch it.

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Even though a reference may be declared as a constant, the reference may point to data which may be changed