What with your earlier post and this one it is not clear what your overall aim is. Could it be that you want to move the timestamp from the front to the middle? Perhaps you could explain more clearly what you are trying to achieve from both nodes.

$ 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
$
[download]

I hope this wild guess is useful.