If the file is so big that you don't have enough disk space available for both the "before" and "after" versions, then you can go to quite a bit of extra work to save disk space: 
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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But be sure to test this code on a subset of your file and make a backup of the file beforehand as a failure of this code will simply leave your file corrupted.

        - tye (but my friends call me "Tye")

In reply to Re: What is the best way to remove(delete) specific lines from a very large file? by tye
in thread What is the best way to remove(delete) specific lines from a very large file? by Flame

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