It may make more sense to prepend the extra string in your accessor.

package Test;
use Moose;

my $prepend = '/some_string/';
has appended => (is => 'rw');

around appended => sub {
    my ($orig, $self) = (shift, shift);
    my $value = $self->$orig(@_);
    return $prepend . $value;
};

1;
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