You could turn the string into an array, using split, and then use grep to check existence of an item. (Or use a hash as a set if you need to test this one list a lot.) 
my @list = split /,/, $thestring;
if(grep { $_ == $thecheck } @list) {
    print "Found it!\n";
}
# Or, if you need this a lot:
# preparation:
my %set;
$set{$_} = 1 foreach split /,/, $thestring;

# actual test:
if($set{$thecheck}) {
    print "Found it!\n";
}

[download]

If you do insist on using a regular expression, you need to be aware that a number can appear at the beginning or at the end of a string. Example check: 

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

[download]
but I prefer the double negative: 
if($thestring =~ /(?<![^,])$thecheck(?![^,])/) { ... }

[download]
(If there's anything in front or right after the match, it may not be anything other than a comma.)

In reply to Re: Checking for occurrence in comma separated string by bart
in thread Checking for occurrence in comma separated string by Delusional

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