use JSON;

sub remove_newlines {
    my $str = shift;
    $str =~ s/^\s+//gsm;
    $str =~ s/\n//gsm;
    return $str;
}

%HoA = ('foo' => [1,2,3], 'bar' => ['a','b','c']);
print "plain:\n".JSON->new->encode(\%HoA)."\n";
my $pretty = JSON->new->pretty->encode(\%HoA);
$pretty =~ s/(\[.*?\])/remove_newlines($1)/egsm;
print "pretty:\n$pretty";