my $so_called_constant = 'unmodified';
print "constant: $so_called_constant\n";
make_modified( $so_called_constant );
print "constant: $so_called_constant\n";
make_modified( 'literal' );

sub make_modified { $_ = 'modified' for @_ }
__END__
constant: unmodified
constant: modified
Modification of a read-only value attempted