You should insert some code tags to make it easier to read. Other than that, your logic seems messed up here.

You might try capturing what you want to keep in a s/// substition, like below (untested):

foreach my $i (@n) {
  
  # strip any white space
  $i =~ s/\s//gm; # is the /m multi-line needed?
  
  # this checks for beginning with 16 digits, plus a matching string.
  # this will not match unless both the digits, and the string (plus 3
+0 chars on each end) 
  # are in your string.
  
  $i =~ s/^(\d{16}).*?(.{30}Some search tring.{30})/$1$2/gm; # again, 
+is the /m needed?
}
[download]

I would highly recommend reading a tutorial, such as

perldoc perlrequick
perldoc perlretut
[download]

Update: fixed typo in my regex