use List::Util qw(min max);
sub rect_overlap {
    my($r1, $r2) = @_;  # two rectangles, like [x1, y1, x2, y2]
    # r1 is on the left of r2
    return 0 if max(@$r1[0, 2]) <= min(@$r2[0, 2]);
    # r2 is on the left of r1
    return 0 if max(@$r2[0, 2]) <= min(@$r1[0, 2]);
    # r1 is below r2
    return 0 if max(@$r1[1, 3]) <= min(@$r2[1, 3]);
    # r2 is below r1
    return 0 if max(@$r2[1, 3]) <= min(@$r1[1, 3]);
    # they must overlap:
    return 1;
}

# some tests:
use Test::Simple 'no_plan';
ok  rect_overlap [10, 10, 30, 30], [20, 20, 40, 60];  # do
ok !rect_overlap [10, 10, 30, 30], [20, 40, 40, 60];  # don't
ok  rect_overlap [10, 10, 30, 30], [15, 15, 20, 20];  # inside