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my @list = split /,/, $thestring;
if(grep { $_ == $thecheck } @list) {
...
if($set{$thecheck}) {
    print "Found it!\n";
}

if($thestring =~ /(?:^|,)$thecheck(?=,|$)/) { ... }

if($thestring =~ /(?<![^,])$thecheck(?![^,])/) { ... }