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

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

use constant FORTYTWO => 42; # note: =>, not =

...
if (FORTYTWO=$answer) { # "Can't modify constant item in scalar assign
+ment"
    ...
}