If you don't need interpolation, put your long command string into an array instead using the quote-words (qw{ ... } in Quote Like Operators) operator which allows you to break the word list over multiple lines.

knoppix@Microknoppix:~$ perl -E '
> @cmd = qw{
>    ps
>    -ef
>    |
>    grep
>    gnome
>    |
>    grep
>    -v
>    grep
>    };
> $res = qx{ @cmd };
> print $res;'
knoppix   3829     1  0 09:40 ?        00:00:01 gnome-terminal
knoppix   3834  3829  0 09:41 ?        00:00:00 gnome-pty-helper
knoppix   3856     1  0 09:41 ?        00:00:30 gnome-terminal
knoppix   3858  3856  0 09:41 ?        00:00:00 gnome-pty-helper
knoppix   4006  4005  0 09:43 ?        00:00:00 gnome-pty-helper
knoppix@Microknoppix:~$
[download]

Even if you do need interpolation you can use this method by using push and breaking your word list up a bit so that interpolated variables go in the gaps.

knoppix@Microknoppix:~$ perl -E '
> $lookFor = q{gnome};
> push @cmd, 
>    qw{
>       ps
>       -ef
>       |
>       grep
>       },
>    $lookFor,
>    qw{
>       |
>       grep
>       -v
>       grep
>       };
> $res = qx{ @cmd };
> print $res;'
knoppix   3829     1  0 09:40 ?        00:00:02 gnome-terminal
knoppix   3834  3829  0 09:41 ?        00:00:00 gnome-pty-helper
knoppix   3856     1  0 09:41 ?        00:00:30 gnome-terminal
knoppix   3858  3856  0 09:41 ?        00:00:00 gnome-pty-helper
knoppix   4006  4005  0 09:43 ?        00:00:00 gnome-pty-helper
knoppix@Microknoppix:~$
[download]

I hope this is of use.