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perl -ne '}{ print $.' # count lines

perl -ne 'BEGIN{$_=shift for$B,$A,$p}/$p/and@l=(),do{<>for 1..$A},next
+;push@l,$_;print shift@l if$B<@l}{print@l' 3 5 pattern test.txt

sub BEGIN {
    ($_ = shift(@ARGV)) foreach (($B, $A, $p));
...
{
    print(@l);
}

keep 1
keep 2
...
delete 5
keep 20
keep 21

keep 1
keep 2
...
keep 19
keep 20
keep 21