Of course, that also works with a junky string:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $s = 'junk%%%first1:::sur1:::middle1%%junk%%first2:::sur2:::middle
+2%%junk%%first3:::sur3:::middle3%%junk';
 ;;
 my @arr = $s =~ /\w+:::\w+:::\w+/g;
 dd \@arr;
"
[
  "first1:::sur1:::middle1",
  "first2:::sur2:::middle2",
  "first3:::sur3:::middle3",
]
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but the OPer doesn't say if he or she is working with a validated string or not.