#!/user/bin/perl -w
use strict;
my $quit = 0;
print "Welcome to the mathq program\n";

until ($quit) {
    print "Enter a number\n";
    my $first_number = <STDIN>;
    chomp ($first_number);    

    if ($first_number eq 'q') {
        $quit = 1;
    } else {
        print "Enter a second number\n";
        my $second_number = <STDIN>;
        ...
    }
}

if ($second_number != 0) {
    ...
} elsif ($second_number == 0) {
    ...
} else {
    print "Invalid input: Type a number or 'q' to quit\n";
}

##</code><code>##

#!/user/bin/perl -w
use strict;
my $quit = 0;
print "Welcome to the mathq program\n";

until ($quit) {
    print "Enter a number\n";
    my $first_number = <STDIN>;
    chomp ($first_number);    

    if ($first_number eq 'q') {
        $quit = 1;
    } else {
        print "Enter a second number\n";
        my $second_number = <STDIN>;
        ...

        if ($second_number != 0) {
            ...
        } elsif ($second_number == 0) {
            ...
        } else {
            print "Invalid input: Type a number or 'q' to quit\n";
        }
    }
}

##</code><code>##

if (...) {
   ...
   last;
} else {
   ...
}

##</code><code>##

if (...) {
   ...
   last;
}

...

##</code><code>##

if (X) {
   ...
} elsif (! X) {
   ...
} else {
   ...
}