Happens to everyone using C or languages inheriting or stealing from C. But you can make the compiler / interpreter complain loudly if you make sure the constant is on the left side of the operator:

if ($answer=42) { # $answer is now always 42
    ...
}


if (42=$answer) { # "Can't modify constant item in scalar assignment"
    ...
}

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This won't help when you compare two variables, but in your case, the variables $an1 and $an2 aren't variable, but constant. So make them constant, either by using Readonly or by using constant:

use Readonly;
Readonly my $fortytwo => 42; # note: =>, not =

...

if ($fortytwo=$answer) { # "Modification of a read-only value attempte
+d"
    ...
}
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use constant FORTYTWO => 42; # note: =>, not =

...

if (FORTYTWO=$answer) { # "Can't modify constant item in scalar assign
+ment"
    ...
}
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Or, in this trivial case, get rid of $an1 and $an2 and use 1 and 2.

I prefer Readonly over constant, because Readonly just makes the variables readonly, with no surprises, whereas constant (ab)uses the perl optimizer and implements the constants as functions returning a constant value. This has some nasty side effects documented in Readonly.

Alexander