That is documented behavior:

NOT A BUG
 • The various patterns are not anchored. That is, a pattern like "$RE
+{num}{int}"
   will match against "abc4def", because a substring of the subject ma
+tches.
   This is by design, and not a bug. If you want the pattern to be anc
+hored, use
   something like:

     my $integer = $RE{num}{int};
     $subj =~ /^$integer$/ and print "Matches!\n";
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