# $y is <C>yet</C> set for the second read, @u is <C>unwanted</C> <P>

perl -lnE "BEGIN{push @ARGV,$ARGV[0]}eof&&!$y?$y=close ARGV:/XXXXX/&&!$y?push @u,$.-1..$.+1:$y&&!($.~~@u)?say:0" next-and-previous.txt
11111
22222
33333
44444<P>


##</code><code>##

perl -lnE "BEGIN{$r=shift;$r=qr/$r/;$n=shift@ARGV;push @ARGV,$ARGV[0]}eof&&!$y?$y=close ARGV:/$r/&&!$y?push @u,$.-$n..$.+$n:$y&&!($.~~@u)?say:0" XXXXX 1 next-and-previous.txt
11111
22222
33333
44444

perl -lnE "BEGIN{$r=shift;$r=qr/$r/;$n=shift;push @ARGV,$ARGV[0]}eof&&!$y?$y=close ARGV:/$r/&&!$y?push @u,$.-$n..$.+$n:$y&&!($.~~@u)?say:0" XXXXX 2 next-and-previous.txt
22222


##</code><code>##

perl -slnE "BEGIN{$r=qr/$r/;push @ARGV,$ARGV[0]}eof&&!$y?$y=close ARGV:/$r&&!$y?push@u,$.-$n..$.+$n:$y&&!($.~~@u)?say:0"  
           -- -r=XXXXX -n=1 next-and-previous.txt

11111
22222
33333
44444