I find it easier to avoid interpolation in that case. You could use contatenation.

    eval('
        package '.$package.';
        sub '.$accessor.' {
            my $t=shift;
            $t->{'.$var.'}=$_[0] if @_;
            $t->{'.$var.'};
        }
    ')
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I prefer sprintf.

    eval(sprintf(
        '
            package %s;
            sub %s {
                my $t=shift;
                $t->{%s}=$_[0] if @_;
                $t->{%s};
            }
        ',
        $package, $accessor, $var, $var
    ))
[download]

Note that I actually build the sub in the right package. That changes some of the meta information associated with the sub (e.g. its __PACKAGE__).

Update: Fixed copy and paste error.