The documentation explains this.

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

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