Your backticks are just fine. The problem is that you're running with warnings turned on and you haven't defined the variable $a_log anywhere.

When you get error messages you don't understand, a good place to read up on them is perldoc perldiag, or better yet, run your script with use diagnostics; near the top.

Oh yes, and read up on scoping (see Dominus' home node for a great link), and add use strict; and declare your variables using my, our (Perl 5.6.0+) or use vars qw($var $names). You'll thank yourself for it later.

Update oh duh. The other replies figured out that you're trying to use $a as the variable and append "_log.txt" to the end of it. You could very well be running under use strict because $a is a special global variable. I'd suggest using something more descriptive anyway, but note that $a and $b have a special meaning in Perl. Ref: perlfunc:sort

perl -e 'print "How sweet does a rose smell? "; chomp ($n = <STDIN>); 
+$rose = "smells sweet to degree $n"; *other_name = *rose; print "$oth
+er_name\n"'
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