The correct method for taking command line parameters into a Perl script is to use the @ARGV array. Your example might look like
in reply to user input file
which the user would invoke as
open(FILE,"<", $ARGV) or die "Open failed for $ARGV: $!";
Note as well that I've added a test to see if the file open succeeded, and added a diagnostic in case it didn't. See Simple Opens in perlopentut.
$ perl script.pl Working_On.csv
#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.