Hi Sunil, If i understand your requirement correclty,

try this, Customer_."$timestmp"._100

Update:

$a=Customer100;
$b=20130201;
$a=~ /(.*?)(\d+)/;
$a= $1."_".$b."_".$2;
print $a;
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$a=~ s/(.*?)(\d+)/$1\_$b\_$2/;
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This depends on the exact format of the filename. But possible the following example will give you an idea:

my $filename = "customer100";
my $timestamp= "02012013";

$filename =~ /(\w+)(\d+)/;              # split filename into "custome
+r" and "100"
$filename = $1."_".$timestamp."_".$2;

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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
$
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I hope this wild guess is useful.

WHY are you doing something that looks like makework? If you need to sort customers by alpha (the only plausible reason for your spec that I've dreamed up), you can do that with the datestamp in its initial position. This 4th line of this code is lifted almost verbatim from the doc for sort

my @old = qw(20120301Customer100 20120402Alsocust200 20130102Nothercu3
+00); 
my (@new, $item);

@new = sort { substr($a, 8) cmp substr($b, 8) } @old;

for $item(@new) {
    say $item;
}
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See perldoc -f sort and perldoc -f substr. But for best results (and as has been asked before) tell us what you really want to do and WHY you think you need to do that.