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 30 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?
}

##</code><code>##

perldoc perlrequick
perldoc perlretut