See this tutorial for a step-by-step example of getting your program to run. Perl is not interactive in that way. You need to save your program in a file and then use the Perl compiler to run it. Or you can run a short program in the command line in this fashion:

perl -e "$a=2; $b = $a + $a; print $b,'\n'"
[download]

$a=2; $b=$a + $a; print $b, "\n";
[download]

• What OS are you using?
• Wrap your code in "code" tags: Writeup Formatting Tips
• perlintro
• Basic debugging checklist

Just to comment on your immediate needs -- after you type (or paste) in your sample program, enter Control-d and then press enter. By pressing Control-d, you are essentially letting the shell know you have entered all the data you wish for the perl interpreter, allowing the interpreter to now run your code.

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

I guess you want it interactive. If so, please try something like this:

karls-mac-mini:~ karl$ perl -d -E 1

Loading DB routines from perl5db.pl version 1.44
Editor support available.

Enter h or 'h h' for help, or 'man perldebug' for more help.

main::(-e:1):    1
  DB<1> $a=2;

  DB<2> $b=$a + $a;

  DB<3> say $b;
4
[download]

Please see perldebug and perlrun for more details.

Best regards, Karl