AoC day 4. This one is silly and horribly inefficient, but it was created from the part 1 solution by only adding a single line. (And the termination logic is cheeky.)

use v5.36;<br><br>my $grid = do { local $/; readline };<br>my $width = index($grid, "\n") + 1;<br><br>my $count = 0;<br>while ($grid =~ /\@/g) {<br>    my $p = $-[0];<br>    my $bad = 0;<br>    for my $o (map +($_, -$_), 1, $width - 1, $width, $width + 1) {<br>        my $q = $p + $o;<br>        $bad++<br>            if 0 <= $q < length($grid)<br>            && substr($grid, $q, 1) eq '@';<br>    }<br>    if ($bad < 4) {<br>        $count++;<br>        substr $grid, $p, 1, '.';<br>    }<br>}<br><br>say $count;<br>

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