Welcome to the Monastery, Fian7. Please, read and try to follow the basic rules: Markup in the Monastery. Also note there is nothing called PERL here, the language is "Perl" and the interpreter is "perl".
Concerning the code, it should have probably been:

unless ($nth =~ /^[+-]?\d+$/ ) { ##Removed this code to make things si
+mpler##
}
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unless means if not (see perlsyn).
The =~ operator matches (see perlop) the variable $nth against a regular expression, enclosed in slashes.
To match, a string can (hence ?) start with a plus or minus sign, then it should have one or more (hence +) digits (\d) and nothing more ($).