perl matching full string - perl

Question

I am very new to Perl. Trying to grep the full line for matched pattern of the string. Seems like it is not able to search for full string. Any suggestion?
use strict;
use warnings;
my $prev;
#my $pattern = ":E: (Sub level extra file/dir checks):";
open(INPUTFILE, "<log_be_sum2.txt") or die "$!";
open(OUTPUTFILE, ">>extract.txt") or die "$!";
while (<INPUTFILE>){
if ($_ =~ /^:E: (Sub level extra file/dir checks):/){
print OUTPUTFILE $prev, $_;
}
$prev = $_;
}

Answer 1

If you want to match a literal / in a regular expression, it either needs to be escaped with a backslash, or you need to use a different character as the regexp quote character (! in the below example). The parenthesis also have to be escaped so they're not treated as a capturing group:
use strict;
use warnings;
my $prev;
#my $pattern = ":E: (Sub level extra file/dir checks):";
open(INPUTFILE, "<", "log_be_sum2.txt") or die "$!";
open(OUTPUTFILE, ">>", "extract.txt") or die "$!";
while (<INPUTFILE>){
if ($_ =~ m!^:E: \(Sub level extra file/dir checks\):!){
print OUTPUTFILE $prev, $_;
}
$prev = $_;
}
Note the change to the three-argument version of open, which is highly recommended. Might consider lexical file handles too. And good for you for using warnings and strict mode! Don't see that enough in new users of the language.

Answer 2

It's good that you are using strict and warnings but you should pay attention to the error/warning messages, and post them along with the question if you don't understand them. My version of Perl fails with an error about an unmatched (. The reason this particular error is thrown is because Perl thinks your regexp is complete when it sees the "/" in "file/dir".
When you have special characters, a good practice is to use quotemeta. I noticed you have a commented line with a variable assignment to pattern. You could uncomment that and use it like this:
...
my $pattern = quotemeta ":E: (Sub level extra file/dir checks):";
...
if ($_ =~ /^$pattern/){
...
}
...
}
But there is also a shortcut documented in perlre: the \Q and \E escape sequences. You can use it like $_ =~ /^\Q$pattern\E/. You can still use it and avoid the variable assignment, but in your case you will need to use a different character for the quote-like operator, since your pattern contains a literal /. I tend to prefer m{}, but it's really up to you as long as it's not /.
use strict;
use warnings;
my $prev = q{}; # NOTE: see NOTE below
open INPUTFILE, "<", "log_be_sum2.txt" or die "$!";
open OUTPUTFILE, ">>", "extract.txt" or die "$!";
while (<INPUTFILE>){
if ($_ =~ m{^\Q:E: \(Sub level extra file/dir checks\):\E}){
print OUTPUTFILE $prev, $_;
}
$prev = $_;
}
*NOTE - I seeded $prev with an empty string, because otherwise if your match is on the first line, you will try to print an undefined value, which will result in a warning.