Or, if GrandFather's solution is too elegant and simple, you can use a module. (It is way overkill here, though!)

use strict;
use Set::Scalar;

my $s = Set::Scalar->new( qw/yes no/ );

my $var = "no";

if ( $s->has($var)){
    print "found $var\n";
}
else{
    print "never heard of $var\n";
}
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