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my $op = shift @ARGV;
if ( $op eq 'add' ) {
...
    my $rtn = multiply( @ARGV );
    print "The product is: $rtn\n";
}

sub add {
    my $sum = 0;
...
    }
    return $sum;
}

sub add {
    my $sum = 0;
    $sum += $_ for @_;
    return $sum;
}

#!/usr/bin/perl --
use strict;
...
);

$ops{$op}(@ARGV) if exists $ops{$op};

#!/usr/bin/perl --
use strict;
...
    ( $result = 1, map { $result *= $_ } @ARGV );
print ( $op eq 'add' ? 'The sum is:' : 'The product is:' );
print " $result\n";