package MyText;

sub init_XS {
    my $class = shift;
    my $usexs = shift;
    if ($usexs) {
        eval {
            use Text::CSV_XS;
            @ISA = ('Text::CSV_XS');
        }
    }
    else {
        eval {
            use Text::CSV;
            @ISA = ('Text::CSV');
        }
    }
}
1;

##</code><code>##


use MyText;

my $usexs = 1;     #decide whether to use XS

MyText->init_XS($usexs);

my $csvobj = MyText->new();   

#Text::CSV_XS->new gets called if $usexs is true.
#Text::CSV->new    gets called if $usexs is false.

#do stuff with $csvobj