Assuming your file isn't too big (i.e will comfortably fit in memory)

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;
[download]

Or if the file is too big to be read entirely into memory

open(my $read,  "<", "your_file") or die("ack: $!");
open(my $write, ">", "tmpfile$$") or die("ack: $!");

while(<$read>) {
  next unless /\b some string\b/;
  print $write;
}

close $read;
close $write;

rename $write, $read;
[download]

See. perlopentut and perlfunc for more info.
HTH

_________
broquaint