Tip #9 from the Basic debugging checklist: Demystify regular expressions by installing and using the CPAN module YAPE::Regex::Explain

(?i-msx:(?:(\d+)\.)+(\d+))

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?i-msx:                 group, but do not capture (case-insensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (?:                      group, but do not capture (1 or more times
                           (matching the most amount possible)):
----------------------------------------------------------------------
    (                        group and capture to \1:
----------------------------------------------------------------------
      \d+                      digits (0-9) (1 or more times
                               (matching the most amount possible))
----------------------------------------------------------------------
    )                        end of \1
----------------------------------------------------------------------
    \.                       '.'
----------------------------------------------------------------------
  )+                       end of grouping
----------------------------------------------------------------------
  (                        group and capture to \2:
----------------------------------------------------------------------
    \d+                      digits (0-9) (1 or more times (matching
                             the most amount possible))
----------------------------------------------------------------------
  )                        end of \2
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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Here is one way to get what you want:

use warnings;
use strict;

$_ = "1.2.17.3 Glückwunschschreiben 12 und";
if ( /(\d+)\.(\d+)\.(\d+)\.(\d+)/ ) {
    print "$1   $2   $3  $4\n";
}

__END__

1   2   17  3
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