Nothing to add to Corions answer, you don't want to use that. But if you still insist or are asking just out of curiosity, this works:

> perl -e '$size_orange=55; my $fruit = "orange"; my $name_var = "size
+_$fruit";print $$name_var; print $size,"\n";'
55
[download]

Note that $size_orange has to be a global variable, it won't work when there is a 'my' before $size_orange. Instead of $$name_var ${$name_var} would also have worked, note the curly braces instead of your parentheses