Thanks Rolf. You are right. The code snippet is as below: 
my @log_patterns = (
qr/ERROR/,
qr/Error/,
qr/FATAL/i,
qr/critical/i,
);

my @log_lines=<$read_tmp_log>;
my $i;
my $j;

for (@log_patterns){
  for $i (0..$#log_lines) {
    next unless $log_lines[$i] =~ $_;
    my $a = $i - 5 < 0 ? 0 : $i - 5;;
    my $b = $i + 5 > $#log_lines ? $#log_lines : $i + 5;
    print $output_file "\n";
    for $j($a..$b){ 
      print $output_file $log_lines[$j]; 
   }
 }
}

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In reply to Re^2: Print line from file only once even if occurrence of pattern is more than once in the line by jayu_rao
in thread Print line from file only once even if occurrence of pattern is more than once in the line by jayu_rao

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