Another way:

c:\@Work\Perl\monks>perl -wMstrict -le
"print 'perl version: ', $];
 ;;
 my @t = qw(aa1.2 aa1_bb2 aa1_2 aa1_2bb 1aa);
 ;;
 my $sep = qr{ [-_] }xms;
 my $num = qr{ \d+  }xms;
 my $ok  = qr{ (?! (?: \A | $sep) $num) . }xms;
 ;;
 for my $s (@t) {
   print $s =~ m{ \A $ok* \z }xms ? '  ' : 'NO', qq{ match: '$s'};
   }
"
perl version: 5.008009
   match: 'aa1.2'
   match: 'aa1_bb2'
NO match: 'aa1_2'
NO match: 'aa1_2bb'
NO match: '1aa'
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