The regular expression can be explained, e.g.:

perl -MYAPE::Regex::Explain -wle ' my $re = qr/^[+-]?\d+$/; 
print  YAPE::Regex::Explain->new($re)->explain();' 

The regular expression:

(?-imsx:^[+-]?\d+$)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  [+-]?                    any character of: '+', '-' (optional
                           (matching the most amount possible))
----------------------------------------------------------------------
  \d+                      digits (0-9) (1 or more times (matching
                           the most amount possible))
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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So, all in all the code checks if the content of $nth contains an integer with or without a prefix (+ or -); if it does not, then the removed code is executed.

Documentation: perlretut perlre

edit: hints about s/PERL/Perl/ and code tags removed, as choroba already mentioned it