Just keep a copy of each line, ready to print if the next one matches.

use strict;
use warnings;

my $last_line='';
open (DIFFR, "/tmp/log.diff") 
    or die "Can't open /tmp/log.diff: $!";
while (<DIFFR>) {
    chomp;
    if (/\bERROR\b/) {
        print CSVFILE $last_line;
        print CSVFILE "$line,";
    }
    $last_line = $_;
}
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