Beside the already mentioned constant pragma, there's also the Readonly module. The advantage of Readonly is that you can mark interpolatable variable readonly (that is, constant), instead of using subroutines. 
use Readonly;
Readonly my $foo => "bar";
$foo .= "baz";
__END__
Modification of a read-only value attempted at ...

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In reply to Re: How can I declare constants? by Perl Mouse
in thread How can I declare constants? by Anonymous Monk

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