use strict;
use warnings;

print "Type in a number: ";
my $x = <>;
chomp($x);
die "ERROR: '$x' is not a number" unless($x =~ /^\d+$/);

print "Type in another number: ";
my $y = <>;
chomp($y);
die "ERROR: '$y' is not a number" unless($y =~ /^\d+$/);

print "What would you like done?\n\nYour choice is:\n1) Add\n2) Subtract\n3) Multiply\n4) Divide\n";
my $ask = <>;
chomp($ask);

if ($ask eq "Add") {
    my $z = $x + $y;
    print "$x + $y = $z";
} else {
    die "ERROR: '$ask' is an unknown operation";
}

##</code><code>##

Type in a number: 4
Type in another number: 5
What would you like done?

Your choice is:
1) Add
2) Subtract
3) Multiply
4) Divide
1
ERROR: '1' is an unknown operation at ./test.pl line 23, <> line 3.