Change
if (\%add($num) { .. }
to
if (add($num)) { .. }

In context, it would actually be:

sub is_image_size_ok {
    my ($image) = @_;
    my $img = get($image);
    my ($height, $width) = imgsize(\$img);
    return $height < $max_h and $width < $max_w;
}

push @array, $string =~ m#stuff#g
    if is_image_size_ok($image);
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