TIMTOWDI or here is one way to use $. ;-)

my $context  = 4;
my @buffer   = ('') x $context;
my $print_to = 0;
my $match    = qr/42/;

while(<DATA>) {
    if ( m/$match/ ) {
        print @buffer;
        $print_to = $. + $context;
        @buffer   = ('') x $context;
    }
    push @buffer,$_; shift @buffer;
    print if $. <= $print_to;
}
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