Try this (some slight changes to see a better circle)
#!/usr/bin/perl
use strict;
use warnings;
use List::Util qw( max min );
my $max = 40;
my @aoa = map { [ ( 'o' ) x ($max + 1) ] } 0..$max ;
display( @aoa );
# @to_change will contain [y1,x1],[y2,x2]...
#my @to_change = illuminate( 5, 4, 6 );
my @to_change = illuminate( $max >> 1, $max >> 1, $max - 2 >> 1 );
print "\n";
$aoa[$_->[0]][$_->[1]] = 'x' for @to_change;
display( @aoa );
sub illuminate
{
my @to_change;
my $center_r = shift;
my $center_c = shift;
my $radius = shift;
#...
for my $row ( 0 .. $#aoa )
{
my $delta_x = eval { int sqrt $radius ** 2 - ($center_r - $row) **
+ 2 };
if( defined $delta_x )
{
my $low = max 0, $center_c - $delta_x;
my $high = min $#{ $aoa[$row] }, $center_c + $delta_x;
push @to_change, map [ $row, $_ ], $low .. $high;
}
}
return @to_change;
}
sub display
{
foreach my $row ( @_ )
{
foreach my $col ( @$row )
{
print $col;
}
print "\n"
}
}