For the specific question, a better option might be to use overload and do dispatch based on the value you get. Something like:

package SomeData;

use overload
    '+' => 'add',
    '-' => 'subtract',
        fallback => 1;

sub add
{
    my ($x, $y) = @_[0..1];
    ($x, $y) = ($y, $x) if $_[2];

    $self->{is_number}
        ? $x + $y
        : $x . $y;
}
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Now, personally, I would never overload + to act as string concatenation. Concatenation isn't commutative, which breaks the implicit contract most people have with addition. As well, how do you define multiplication for strings?