Here some code which looks a bit cryptic but check this node, perldoc perlrun and perldoc perlvar and it should be become clear.

{
   local $^I = '';
   local @ARGV = ($filename);
   while (<>) {
       next if CRITERIA_TO_SKIP_LINE;
       print;
   }
}
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cat big-file.txt | grep -v 'bad line' > new-file.txt
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Or, alternatively:

perl -i.orig -pe '$_="" if /bad line/' big-file.txt
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my $fileName= "toBeTrimmed.txt";
open( FILE, "+< $fileName" )
   or  die "Can't update $fileName: $!\n";
my $readPos= tell(FILE);
my $writePos;
while(  <FILE>  ) {
    if(  wantLine($_)  ) {
        $readPos= tell(FILE)
            or  die "Can't tell after read: $!\n";
        if(  defined($writePos)  ) {
            seek( FILE, $writePos, 0 )
                or  die "Can't seek to write: $!\n";
            print FILE $_
                or  die "Failed writing to file: $!\n";
            $writePos= tell(FILE)
                or  die "Can't tell after write: $!\n";
            seek( FILE, $readPos, 0 )
                or  die "Can't seek to read: $!\n";
        }
    } elsif(  ! defined($writePos)  ) {
        $writePos= $readPos;
    }
}
if(  defined($writePos)  ) {
    truncate( FILE, $writePos )
        or  warn "Can't truncate: $!\n";
}
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Ah. The post with a succinct question!

Read from file A, write anything you don't want to delete to file B. If the files are really large, it'll go faster to have the files on separate disk drives. Moving disk heads is relatively expensive. You can minimize head contention by reading from one head (file) while writing to a separate head (file). Unless, of course, the source file is fragmented, or the target disk is heavily fragmented.

use Tie::File;

tie @array, 'Tie::File', "filename" or die "something is wrong: $! \n"
+;
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#!/usr/bin/perl

my $FILE = $ARGV[0];
my $MAXSIZE=10;

exit -1 if ! -f $FILE;

$num_lines = 0;
open FILE, ">$FILE";
for (;;) {
        chomp($mytime=`date +%T`);
        my $line = "$mytime";
        $line .= ";" . sprintf "%.2f", rand(10);
        $line .= ";" . sprintf "%.2f", rand(20);
        $line .= ";" . sprintf "%.2f", rand(30);
        $line .= ";" . sprintf "%.2f", rand(40);
        if($num_lines < $MAXSIZE){
                push @array, "$line\n";
                $num_lines++;
        }else{
                shift @array;
                push @array, "$line\n";
        }
        #Get the end position of the file
        seek(FILE, 0, 2);
        $endpos = tell(FILE);

        seek(FILE, 0, 0);#Seek to start
        print FILE @array;

        #For the rest of the bytes print " "
        $curpos = tell(FILE);
        for(my $cnt=$curpos;$cnt<$endpos;$cnt++){
                print FILE " ";
        }

        sleep 1;
}
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