package Foo;

use strict;
use overload '""' => \&interp;

sub new {
    my $class = shift;
    bless {
        str => $_[0],
        re  => qr/^$_[0]$/,
    }, $class;
}

sub as_string {
    $_[0]->{str};
}

sub re {
    $_[0]->{re};
}

sub interp {
    my $self = shift;
    my $condition = 1;
        # 1 if called in qr{} context
        # 0 otherwise
    $condition ? $self->re : $self->as_string;
}
1;