Using || is less painful than defined($1)?...

sub munge {
    my $str = shift;

    $str =~ s/^R(?:([12])|(3))?$/(${1}||"")."text".($2||"")/e;
    $str;
}
[download]

Other than that, I don't know of other more elegant options.

Edit:Anonymous Monk below is correct - _if_ our regex could capture a zero for $1 or $2, we'd have to do the defined()?: approach. But 0 can't match either capture here...