You are going to have to open the file and process each item line by line.

$input = "oldlog";
my $output = "newlog";
open (INPUT,"<$input") || die "$!\n";
open (OUTPUT,">$output") || die "$!\n";
foreach $line (<INPUT>) {
    chomp($line);              # remove newlines or whatever regex
    # do line-by-line processing.
print OUTPUT $line;
}
close INPUT;
close OUTPUT;
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This will open your log file, do something to it line by line and dump the results in another logfile. The regex stuff is the hard part.