Not that I'm disagreeing with all the errors/problems other people have pointed out, but this is really the canonical case for using a dispatch table.

#!/usr/bin/perl -w
use strict;

my $op = shift or die "You must specify an operation followed by numbe
+rs.\n";
die "Non-numeric input.\n" if grep /\D/, @ARGV;

my %perform = (
    add         =>  sub { local $" = "+"; eval "@ARGV"; },
    multiply    =>  sub { local $" = "*"; eval "@ARGV"; },
);

die "Invalid operation.\n" unless defined $perform{$op};

print $perform{$op}->();
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