$ perl -Mstrict -Mwarnings -E '
> my $filename = q{20130101Customer100.imp};
> say $filename;
> substr( $filename, 8, 0 ) = do {
>    my $ts = substr $filename, 0, 8, q{};
>    qq{_${ts}_};
>    };
> say $filename;'
20130101Customer100.imp
Customer_20130101_100.imp
$