Picking a specific line with a specific string - perl

Question

I am trying this in Perl to pick one complete line from whole document which contains "CURRENT_RUN_ID". I have been using below code to accomplish the above said task but I am unable to enter the while loop.
my $sSuccessString = "CURRENT_RUN_ID";
open(LOG, "$slogfile") or die("Can't open $slogfile\n");
my $sLines;
{
local $/ = undef;
$sLines=<LOG>;
}
my $spool = 0;
my #matchingLines;
while (<LOG>)
{
print OUTLOG "in while loop\n";
if (m/$sSuccessString/i) {
print OUTLOG "in if loop\n";
$spool = 1;
print map { "$_ \n" } #matchingLines;
#matchingLines = ();
}
if ($spool) {
push (#matchingLines, $_);
}
}

Answer 1

You are already done reading from the filehandle LOG after you have slurped it into $sLines. <LOG> in the head of the while will return undef because it has reached eof. You either have to use that variable $sLines in your while loop or get rid of it. You're not using it anyway.
If you only want to print the line that matches, all you need to do is this:
use strict;
use warnings;
open my $fh_in, '<', 'input_file' or die $!;
open my $fh_out '>', 'output_file' or die $!;
while (my $line = <$fh_in>) {
print $fh_out $line if $line =~ m/CURRENT_RUN_ID/;
}
close $fh_in;
close $fh_out;

Answer 2

When you execute this code:
$sLines=<LOG>;
it reads all of the data from LOG into $sLines and it leaves the file pointer for LOG at the end of the file. So when you next try to read from that file handle with:
while (<LOG>)
nothing is returned as there is no more data to read.
If you want to read the file twice, then you will need to use the seek() function to reset the file pointer before your second read.
seek LOG, 0, 0;
But, given that you never do anything with $sLines I suspect that you can probably just remove that whole section of the code.
The whole thing with $spool and #matchingLines seems strange too. What were you trying to achieve there?
I think your code can be simplified to just:
my $sSuccessString = "CURRENT_RUN_ID";
open(LOG, $slogfile) or die("Can't open $slogfile\n");
while (<LOG>) {
print OUTLOG if /$sSuccessString/i/;
}
Personally, I'd make it even simpler, by reading from STDIN and writing to STDOUT.
my $sSuccessString = 'CURRENT_RUN_ID';
while (<>) {
print if /$sSuccessString/i/;
}
And then using Unix I/O redirection to connect up the correct files.
$ ./this_filter.pl < your_input.log > your_output.log