my @customerarray = ( ... );

my $now = time;
my @keep = grep {
    my ($timestamp) = (split '::', $_)[2];
    $now - $timestamp <= 15*24*60*60  # items less than 15 days old
  } @customerarray;

open FH, '>', "$cuslist.tmp" or die "Can't open $cuslist.tmp: $!";
print FH "$_\n" for @keep;
close FH;