open(my $fh, "<+", "your_file") or die("ack: $!");
my @data = <$fh>;
seek $fh, 0, 0;

my $newsize = 0;
for(@data) {
  next unless /\bsome string\b/;
  print $fh;
  $newsize += length;
}

truncate $fh, $newsize;
close $fh;