Note that the lines could occur immediately after one another and I would like to have output print only once in such cases.

To avoid this, a solution using vec could work for you.

The line of code vec($vec, $_, 1) ||= 1 for $lo .. $hi; would assign a 1 to each position. So you wouldn't repeat a line when printing out the results. 

my @log_lines=<$read_tmp_log>;
my $vec = '';

for my $i (0..$#log_lines) {
    next unless lc($log_lines[$i]) =~ /error|fatal|critical/;
    my $lo = $i - 5 < 0 ? 0 : $i - 5;;
    my $hi = $i + 5 > $#log_lines ? $#log_lines : $i + 5;
    vec($vec, $_, 1) ||= 1 for $lo .. $hi;
}

my $newline_needed = 0;
for my $i (0 .. $#log_lines) {
    if (vec($vec, $i, 1) == 1) {
        print $log_lines[$i];
        $newline_needed ||= 1;    
    }
    else {
        print "\n" if $newline_needed;    
        $newline_needed = 0;
    }
}

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In reply to Re^5: Print line from file only once even if occurrence of pattern is more than once in the line by Cristoforo
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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