if ($comment1 =~ " ") { # reads: if comment1 *HAS* a space
  ...
}
if ($comment1 !~ " ") { # reads: if comment1 *DOESN'T HAVE* a space
  ...
}
if ($comment1 eq " ") { # reads: if comment1 *IS* a space
  ...
}
if ($comment1 ne " ") { # reads: if comment1 *IS NOT* a space
  ...
}
if ($comment1 == " ") { # reads: if comment1 *Numerically Equals* a sp
+ace
                        # since space == 0, 
                        # ==> same as 
                        # if($comment1 == 0){ ... }
  ...
}
if ($comment1 != " ") { # reads: *Numerical* not equal to... 
  ...
}
[download]

Hope this helps,,,

PS: As Marza suggested, the llama book is a good book.

Update: Added the other cases.